DOM Articles

Sort by:


  1. Animating with javascript: from setInterval to requestAnimationFrame

    Animating DOM elements[1] or the content of a canvas is a classical use case for setInterval. But the interval is not as reliable as it seems, and a more suitable API is now available…

    Animating with setInterval

    To animate an element moving 400 pixels on the right with javascript, the basic thing to do is to move it 10 pixels at a time on a regular interval.

    JSFiddle demo.

    An HTML5 game based on this logic would normally run at ~60fps[2], but if the animations were too complex or running on a low-spec. device (a mobile phone for instance) and processing a frame were taking more than 16ms, then the game would run at a lower framerate: when processing 1 frame takes 33ms, the game runs at 30fps and game elements move twice as slowly as they should. Animations would still look smooth enough, but the game experience would be altered.

    Animating at constant speed

    To animate at constant speed, we need to calculate the time delta since the last frame and move the element proportionally.

    Animating with requestAnimationFrame

    Since the interval parameter is irrelevant in complex animations, as there’s no guarantee that it will be honored, a new API has been designed: requestAnimationFrame. It’s simply a way to tell the browser “before drawing the next frame on the screen, execute this game logic/animation processing”. The browser is in charge of choosing the best moment to execute the code, which results in a more efficient use of resources[3].

    Here’s how an animation with requestAnimationFrame would be written.
    Note: Following code snippets don’t include feature detections and workarounds necessary to work in current browsers. Should you want to play with them, you should try the ready-to-use animLoop.js.

    Dealing with inactive tabs

    requestAnimationFrame was built with another benefit in mind: letting the browser choose the best frame interval allows to have a long interval in inactive tabs. Users could play a CPU intensive game, then open a new tab or minimize the window, and the game would pause[4], leaving resources available for other tasks.
    Note: the potential impact of such behavior on resource and battery usage is so positive that browser vendors decided to adopt it for setTimeout and setInterval as well[5].

    This behavior also means that the calculated time delta might be really high when switching back to a tab containing an animation. This will result in animation appearing to jump or creating “wormholes[6], as illustrated here.

    Wormholes can be fixed by clamping the time delta to a maximum value, or not rendering a frame when the time delta is too high.

    JSFiddle demo.

    Problems with animation queues

    Libraries such as jQuery queue animations on elements to execute them one after the other. This queue is generally only used for animations that are purposefully consecutive.
    But if animations are triggered by a timer, the queue might grow without bound in inactive tabs, as paused animations stack up in the queue. When switching back to affected tabs, a user will see a large number of animations playing consecutively when only one should happen on a regular interval:

    JSFiddle demo.

    This problem is visible in some auto-playing slideshows such as To work around it, developers can empty animation queues before triggering new animations[7].
    JSFiddle demo.


    The delays of setTimeout and setInterval and of course requestAnimationFrame are unpredictable and much longer in inactive tabs. These facts should be taken into account not only when writing animation logic, but in fps counters, time countdowns, and everywhere time measurement is crucial.

    [1] The DOM can now be animated with CSS3 Transitions and CSS3 Animations.
    [2] 1 frame every 16ms is 62.5 frames per second.
    [3] See the illustration of this fact on msdn.
    [4] The behavior of requestAnimationFrame in inactive tabs is still being worked on at the w3c, and might differ in other browsers.
    [5] See related Firefox bug and related chromium bug.
    [6] This term was first coined by Seth Ladd in his “Intro to HTML5 Game Development” talk.
    [7] See documentation of your js library, such as effects and stop() for jQuery.

  2. A call for quality HTML5 demo markup

    HTML5 is a necessary evolution to make the web better. Before the HTML5 specs were created we used (and still use) a hacked together bunch of systems meant for describing and linking documents to create applications. We use generic elements to simulate rich interaction modules used in desktop development and we make assumptions as to what the user agent (browser) can do for the end user.

    The good thing about this mess is that it taught us over the last few years to be paranoid in our development approach – we realised that in order to deliver working, accessible, maintainable and scalable systems we have to be professional and intelligent in our decisions and especially in our planning and architecting.

    The trouble is that with the excitement around the cool new HTML5 things to play with a lot of these principles get lost and discarded as outdated. They aren’t – part of the mess that the web is in is that in 1999 a lot of people discarded everything else and instead worked only on the new hotness that was Internet Explorer 6 and DHTML. Let’s not repeat that mistake.

    The two faces of HTML5 enthusiasm

    Right now there are two things to get really excited about in HTML5: the richer, more meaningful semantics of new elements and the APIs that give us deep access into the workings of the browser and the operating system. The former allows us to build much richer interfaces and the latter allows us to build much more complex applications.

    All of this comes with the awesome of view source (or check in development tools) for debugging. You can advocate HTML5 by writing clean and useful markup. You can kill it by treating the markup as a second class citizen which is only there to serve the script.

    The markup enthusiasts are very happy about HTML5 and make it easy for people to upgrade – lots of cool new blog templates and boilerplate HTML templates are being built and polyfills created to allow using the new features and not leave behind outdated browsers.

    On the app development side of things it looks much different and that is just sad. My only possible explanation is that people who come from desktop environments now tackle the HTML5 APIs without ever having to care about markup. The pragmatism of HTML5 allows a much more forgiving syntax than XHTML but it shouldn’t mean that we can just re-apply all the bad mistakes that held us back when it comes to maintenance for years.

    During my career as a web developer I realised a few things make a lot of sense when building web apps:

    • If there is an element for a certain task – use that one. It is very likely that the element comes with accessibility and interaction features for free that you would otherwise have to simulate.
    • Separate CSS, JavaScript and HTML – which means it is easy to refactor your code without having to change all of them. It also means you can work in parallel with others instead of breaking each other’s code
    • Never rely on markup or content – as sooner or later some editing tool will come into place that messes with everything you created

    This means a lot of things:

    • For starters it means that inline styles are simply evil as they override any settings you have in your style sheets. Only use them when this is exactly what you need to do or when you calculate them dynamically.
    • The same applies to inline scripting. If you have an onclick="foo()" somewhere in your HTML and foo() changes to bar() you have to rename it in every HTML document (of course nowadays it is one template, but it still means hunting down a reference you might miss)
    • If instead of using a native HTML element for a certain job you use SPANs and DIVs you’ll have to add classes to make them look and work – and simulate the keyboard accessibility, too.
    • You can’t rely on the text value of any element. A <button>edit</button> using the “edit” as the trigger for certain functionality would have to have the JS localised, too when you create the German <button>bearbeiten</button>.

    Bla bla bla… C’mon Chris, it isn’t that bad!

    The above best practices have been mentioned for years and a lot of people get sick of seeing them repeated. After all, this is intelligent development and standard practice in backend technologies. I came across a lot of “creative” uses lately though when doing a view-source on HTML5 demos – specifically the ones in the HTML5 advent calendar. And here is my yield of what people do.

    Simulating a navigation list

    One of the first things I encountered was a painting tool that had a gallery of created graphics as a background. Now, to me this would be a simple list:

        <a href="{link}">
          <img src="{src}" alt="{title}">

    The markup I found though was this:

    <div id="sky">
      <div class="skyTile" style="{calculated styles};display:block">
        <img class="skyThumb" src="{src}" style="{generated styles}">
        <img src="images/thumb_dropShadow.png"
             style="{generated styles}">

    This, of course is generated by the backend. My first gripe is the dropshadow image, after all this is an HTML5 showcase – just use CSS3. We also have the three instances of generated styles and double classes. Granted, an extra class gives you a handle to all images instead of tiles, so why not. But as there is no real link around the image, the click handler has to read the url from somewhere. There is a lot of unnecessary back and forth between DOM and scripting here which does slow down the whole demo. Seeing that this is also the main navigation from the entry page to the editor this could be a list inside a nav element. A list groups these items together, a bunch of DIVs doesn’t (screen readers for example tell you how many items there are in a list).

    Another list I found was supposed to be links to refresh the app and have a hierarchy but instead was a flat list with classes to define hierarchy and group:

    <ul id="mitems">
      <li class="hedtop">Simple</li>
      <li class="mol">{name}</li>
      [... repeated ...]
      <li class="hed">{parent name}</li>
      <li class="mol">{name}</li>
      [... repeated ...]

    This could be OK, granted you also shift keyboard focus, but why not:

    <ul id="mitems">
      <li>{parent name}
          <li><a href="{url}">{name}</a></li>
          [... repeated ...]
      [... repeated ...]

    This would give you styling hooks and functionality for free. Links and buttons are great to trigger functionality – but it seems that is too easy.

    Click (probably) here for functionality

    If I build a button for an application to trigger a certain functionality this is the markup:

    <button type="button" class="edit">Edit</button>

    Buttons are what trigger functionality – both a backend script or a JavaScript. They are keyboard accessible, have a disabled state and sensible out-of-the-box styling that nowadays can be completely overwritten. The class can be the definition of what the button should do – instead of the text which will change. You could use an ID but a class allows to repeat buttons (for example on the top and the bottom of a results list).

    The buttons I found though were slightly different:

    <div id="homeButtonPanel">
      <div class="homeButton" id="drawHomeButton"
           style="display: block;">
        <p>Start Drawing</p>
      <div class="homeButton" id="viewHomeButton"
           style="display: block;">
        <p>View the Mural</p>
    <div id="controlPanel" style="display: block;">
    <div class="titleButton"><p>{title}</p></div>
      <div class="mainMenu">
        <div class="mainButton mainSelected" id="drawButton">
          <div id="inkDisplay">
            <div id="inkMeter" style="width: 94px;">
        <div class="menuContainer">
          <div class="menuButton drawMenuButton" id="behaviorsButton"
               style="display: block; opacity: 1;">
            <p>Gesture<br />Behaviors</p>
        <div class="menuContainer">
          <div class="menuButton drawMenuButton" id="artworkButton"
               style="display: block; opacity: 1;">
        <div class="menuContainer">
          <div class="menuButton drawMenuButton" id="resetButton"
               style="display: block; opacity: 1;">
        <div class="menuContainer">
          <div class="menuButton drawMenuButton" id="undoButton"
               style="display: block; opacity: 1;">
        <div class="menuContainer">
          <div class="menuButton drawMenuButton" id="saveButton"
               style="display: block; opacity: 1;">
      <div class="mainMenu">
        <div class="mainButton" id="viewButton"><p>View</p></div>
      <div class="mainMenu">
        <div class="secondaryButton" id="helpButton"><p>Help</p></div>
      <div class="mainMenu">
        <div class="mainShare shareButtonSmall" id="mainTwitterButton">
          <img alt="twitter" src="images/twitter_icon.png"
               style="opacity: 0.6;">
        <div class="mainShare shareButtonLarge" id="mainFacebookButton">
          <img alt="facebook" src="images/facebook_icon.png">
        <div class="mainShare shareButtonSmall" id="mainEmailButton">
          <img alt="email" src="images/email_icon.png" style="opacity: 0.6;">

    So instead of using a nested list with classes for each button and the hierarchy in the nesting we have a lot of classes and a hand-rolled DIV construct. Instead of making buttons really disabled we rely on opacity and there is quite a mix of inline images and background images (if all were background images, they could be one sprite!). Keyboard navigation will have to be written for this and if you were to add a button you’d have to come up with another ID.

    HTML5 actually has a construct for this. There is a menu element with command child elements. These are in use in Chrome’s side bar for example and should be what we use. If you want to make it work for everyone, a nested list with button elements is what to go for.

    The overly complex DIV construct is quite used though – this was another example:

    <div class="uiScreen" id="startScreen">
      <div class="panelBackground" id="mainResumeButton" style="display: none;">
        <div class="largeUIButton" id="resumeButton">Resume Game</div>
      <div class="panelBackground" id="mainStartButton">
        <div class="largeUIButton" id="startButton">Start Game</div>
      <div class="panelBackground" id="mainHelpButton">
        <div class="largeUIButton" id="helpButton">Help</div>
      <div class="panelBackground" id="mainHighscoreButton">
        <div class="largeUIButton" id="highscoreButton">High Scores</div>

    When in doubt – add an ID and class to everything.

    Other buttons I encountered were actually links pointing to javascript:// using an inline style to indicate hierarchy:

    <ul class="navlist">
      <li><a id="play" href="javascript://"
             style="display: inline;">Play</a></li>
      <li><a id="pause" href="javascript://">Pause</a></li>
      <li style="padding-left: 2px;">Effects:</li>
      <li><a id="bw" href="javascript://">Bw</a></li>
      <li><a id="outline" href="javascript://">Outline</a></li>
      <li><a id="color" href="javascript://">Color</a></li>
      <li><a id="invert" href="javascript://">Invert</a></li>
      <li><a id="matrix" href="javascript://">Matrix</a></li>
      <li><a id="old" href="javascript://">Old</a></li>

    Talking of inline – here’s a great example of a tool generating a lot of code that could be replaced by a single event handler and event delegation:

    <div id="tools">
      <span onmouseout="buttonOut(1)" onmouseover="buttonOver(1)"
            onclick="buttonClicked(1)" id="button1" class="button">
        <img alt="" src="image/button/1.png">
      <span onmouseout="buttonOut(2)" onmouseover="buttonOver(2)"
            onclick="buttonClicked(2)" id="button2" class="button">
            <img alt="" src="image/button/2.png">
      [...repeated 20 times...]
    <div id="toolsSeparator">&nbsp;</div>
      <a title="" id="toolbarButtonSave" class="toolbarButton"
        <img alt="" src="image/save.png">
      <a title="New" id="toolbarButtonNew" class="toolbarButton"
        <img alt="New" src="image/new.png"></a>
      <!--[if !IE]><![IGNORE[--><!--[IGNORE[]]-->
      <a id="toolbarButtonMenu" class="toolbarButton"
         onmouseout="closeMenuDelay()" onmouseover="showMenuHover()"
        <img alt="&gt;" src="image/menu.png">
      <div onmouseout="closeMenuDelay()" onmouseover="overMenu()" id="menu">
        <a class="saveMenu" onmouseout="closeMenuDelay()"
           onmouseover="overMenu()" href="javascript:saveCanvas()">
          save normal
        <a class="saveMenu" onmouseout="closeMenuDelay()"
           onmouseover="overMenu()" href="javascript:saveCanvasHi()">
          save high quality
          <span class="footnote">&nbsp; (rename to *.png)</span>
        <a onmouseout="closeMenuDelay()" onmouseover="overMenu()"
        <a class="lastMenu" target="_top" onmouseout="closeMenuDelay()"
          <span class="footnote">
          <em>a sound memory game</em>

    Notice that if the images for the button couldn’t be loaded for one reason or another (or you can’t see them at all) this application is very confusing indeed – no alternative text for the images and no text content to fall back to. I am also very much sure that the in and out handlers trigger visual effects CSS could handle better.

    Reasons and effects

    I know there are probably good reasons for all of this, and I am sure I will also do things wrongly when I am rushed or want to get things out quickly. What we have to understand though is that right now we have a responsibility to show the best of breed demos we can.

    We cannot preach the open web and technologies and view-source over closed systems and at the same time forget the things we learnt in the last decade. Some of these things I found look like code Frontpage or Dreamweaver could have produced in the 90ies and resulted in a lot of badly performing, hard to maintain products that either still annoy people who have to use them or get replaced every 2 years.

    We have a mandate to educate the new developers coming to the web. Unlearning something is much harder than learning it – so let’s not start with bloat and quick fixes that work but forget to advocate clean code and thinking about the impact your product has on the end users (thinking accessibility) and the poor sods that will have to maintain your product when you are gone. We are not here to advocate effects and products, we are here to promote the tools that allow anyone to easily build something cool that is also easy to understand.

    HTML5 is about evolving the web as a platform – we also need to evolve with it and take more responsibility. We have app and we have markup enthusiasts. Let’s make them work together to build things that are great functionality and clean semantics.

  3. HTML5 adoption stories: and html5 drag and drop

    This is a guest post from Tomas Barreto, a developer who works at They recently adopted HTML5 drag and drop as a way to share files with other people using new features in Firefox. The included video is a pitch for the feature and service, but shows how easy it is to do simple HTML5-based upload progress even with multiple files. Tomas gives an overview of the relatively simple JavaScript required to do this, and how improvements in Firefox 4 will make things even easier. Also have a quick look at the bottom of the post for links to additional docs and resources.

    At, we’re always exploring new ways to help users get content quickly and securely onto our cloud content management platform. So when asked, “What feature would make you use Box more?” during the Box Hack Olympics in April, my colleague CJ and I decided to tackle the most intuitive way to upload files: simply dragging them from the desktop into Box.

    We considered technologies ranging from Gears to Firefox plugins, but only HTML5 had sufficient adoption. By using some of the JavaScript APIs defined in the HTML5 standard, CJ and I could create a seamless drag and drop experience for our users on supporting browsers. Furthermore, using an HTML5-based upload feature would allow us to enable users to select multiple files at once, and also display progress on the client without polling. And with HTML5 adoption across the latest versions of three of the top four browsers, we felt confident about building an upload method based on this new technology without the trade-offs of using a third-party plug-in.

    We rolled out the first rev of our drag and drop feature a few weeks ago, and we’re impressed with how quickly it has been adopted. It’s already one of the most popular ways to get files onto Box, and in its first week it surpassed our old AJAX upload method. You can check out our demo video to get a feel for the feature:

    To build this feature, we referenced a handful of online examples that explained how to use Firefox 3 FileReader object and the drag and drop file event support. Our first implementation used this object to load the file into memory and then took advantage of the latest XMLHttpRequest events to track progress on the client.

    var files = event.originalEvent.dataTransfer.files; // drop event
    var reader = new FileReader();
    reader.onload = function(event) {
      var file_contents =;
      var request = new XMLHttpRequest();
      ... // attach event listeners to monitor progress and detect errors
      var post_body = '';
      .. // build post body
      post_body += file_contents;
      .. // finish post body
      var url = '';
      var request = new XMLHttpRequest();
  "POST",  url, true); // open asynchronous post request
      request.setRequestHeader('content-type', 'multipart/form-data; boundary=""'); // make sure to set a boundary

    This approach worked well because we could use the same server processing code that we previously used for uploads. The main disadvantage here is that the FileReader object reads the entire file into memory, which is not optimal for a general upload use case. Our current HTML5 implementation uses this logic and has forced us to restrict drag and drop uploads to just 25mb. However, thanks to recommendations from the Mozilla team, we’ll be taking an alternative approach for V2 of drag and drop, where the file is read chunks as needed by the request. Here’s how we’re going to do it:

    var files = event.originalEvent.dataTransfer.files; // drop event
    var url = '';
    var request = new XMLHttpRequest();"POST",  url, true); // open asynchronous post request

    Since this approach is not formatted as a multipart form-data, it will require some adjustments on our back-end to support receiving file uploads in this way. However, it’s definitely worth the trade-off since we’ll get all the benefits of the previous method and we don’t need special file size restrictions. In the future, we’ll consider using yet another way to efficiently upload files that is supported in Firefox 4 and uses the traditional multi-part form:

    var files = event.originalEvent.dataTransfer.files; // drop event
    var url = '';
    var request = new XMLHttpRequest();
    var fd = new FormData;
    fd.append("myFile", files[0]);
"POST",  url, true); // open asynchronous post request

    We’re already exploring more ways to enrich the Box experience using HTML5. With HTML5, we can build faster, richer and more interactive features with native browser support, and bridge the traditional gap between desktop software and web applications. Here are just a few cool new upload-related features on our roadmap:

    • Pause/Resume uploads using the Blob slice API to split files into chunks (this will be a huge robustness boost, especially for large uploads)
    • Allowing uploads to resume even after the browser closes by caching the file using IndexedDB support (possibly in Firefox 4)

    We’d also like to begin a discussion about supporting the reverse drag and drop use case: dragging files from the browser to the desktop. Based on our users’ enthusiasm around the drag and drop upload feature, we think the reverse functionality would well received. If you are interested in contributing to a specification for this feature, please let us know (html5 [-at$]!


  4. Firefox 4: Better performance with Lazy Frame Construction

    This is a re-post from Timothy Nikkel’s blog.

    Lazy Frame Construction is new to Gecko and allows many DOM operations (appendChild, insertBefore, etc) to not trigger immediate reflows. This can vastly improve the interactive performance of very complex web pages. If you want to test this out, you should get a Firefox Nightly.

    Lazy frame construction recently landed on mozilla-central. To explain what this means and how this improves things we need some background. Each node in the DOM tree of a webpage has a frame created for it that is used to determine where on the page the node is drawn and its size. A frame corresponds closely to the concept of a box from the CSS spec. We used to create frames for DOM nodes eagerly; that is as soon as a node was inserted into the document we would create a frame for it. But this can create wasted effort in many situations. For example if a script inserts a large number of nodes into the DOM we would create a frame for each node when it is inserted. But with lazy frame construction we can process all those nodes at once in a big batch, saving overhead. Furthermore the time it takes to create those frames no longer blocks that script, so the script can go and do what it needs to and the frames will get created when they are needed. There are other situations where a script would insert nodes into the document and remove them immediately, so there is no need to ever create a frame for these as they would never be painted on screen.

    So now when a node is inserted into a document the node is flagged for needing a frame created for it, and then the next time the refresh driver notifies (currently at 20 ms intervals) the frame is created. The refresh driver is also what drives reflow of webpages and CSS & SVG animations.

    Let’s look at two examples where lazy frame construction helps.

    In this example we insert 80000 div elements and then we flush all pending layout to time how long it takes before the changes made by the script are done and visible to the user. The script can continue executing without flushing layout, but we do it here to measure how long the actual work takes.

    var stime = new Date();
    var container = document.getElementById("container");
    var lastchild = document.getElementById("lastchild");
    for (var i = 0; i < 80000; i++) {
      var div = document.createElement("div");
      container.insertBefore(div, lastchild);
    document.documentElement.offsetLeft; // flush layout
    var now = new Date();
    var millisecondselapsed = (now.getTime() - stime.getTime());

    With lazy frame construction we are able to process the insertion of all 80000 div elements in one operation, saving the overhead of 80000 different inserts. In a build without lazy frame construction I get an average time of 1358 ms, with lazy frame construction I get 777 ms.

    This example comes from a real webpage. We append a div and then set “ = ‘absolute’;”, and repeat that 2000 times, and then we flush all pending layout to time how long it takes before the changes made by the script are done and visible to the user.

    var stime = new Date();
    var container = document.getElementById("container2");
    for (var i = 0; i < 2000; i++) {
      var div = document.createElement("div");
      container.appendChild(div); = "absolute";
    document.documentElement.offsetLeft; // flush layout
    var now = new Date();
    var millisecondselapsed = (now.getTime() - stime.getTime());

    With lazy frame construction we don't even bother creating the frame for the div until after the position has been set to absolute, so we don't waste any effort. In a build without lazy frame construction I get an average time of 4730 ms, with lazy frame construction I get 130 ms.

  5. multiple file input in Firefox 3.6

    Firefox 3.6 supports multiple file input. This new capability allows you to get several files as input at once, using standard technologies. This is a big improvement, since you used to be constrained to one file at a time, or needed to use a third party (proprietary) application. This will be particularly useful, for example, for photo uploads.

    The input tag

    To let your user select a local file, use the input tag on your Web page. This will show the file picker to the user:

    <input type="file"/>

    In Firefox 3.6, the input tag has been expanded to support multiple files:

    <input type="file" multiple=""/>

    The user will still see the same file picker, but will be able to select more than one file.

    The form tag

    You can still use the classic form mechanism:

    <form method="post" action="upload.php" enctype="multipart/form-data">
      <input name='uploads[]' type="file" multiple=""/>
      <input type="submit" value="Send">

    If the server side code is in PHP, don’t forget to make sure that the value of the name attribute has brackets. The brackets are not from the HTML specification, but are required to manipulate the result of the request as an array (see PHP documentation).

    Here’s an example, which goes through the file list and prints each file name:

    foreach ($_FILES['uploads']['name'] as $filename) {
        echo '<li>' . $filename . '</li>';

    Using File API

    Firefox 3.6 also supports FileAPI. This allows you to do extra processing on the client slide before sending the files to the server. You can access the selected files with the files property of the input DOM element and then manipulate the files using the FileAPI.

    For example, here’s how to get the name of each file selected by the user. This is done on the client side, unlike the previous PHP example.

      var input = document.querySelector("input[type='file']");
      // You've selected input.files.length files
      for (var i = 0; i < input.files.length; i++) {
        // input.files[i] is a file object
        var li = document.createElement("li");
        li.innerHTML = input.files[i].name;


    See this mechanism in action in our multiple file input demo. You’ll need Firefox 3.6 (beta).


    To learn more about multiple file input, check out the documentation on MDC.

  6. arun talks about html5, fonts and india

    Recently Arun Ranganathan, one of the members of the Mozilla Evangelism team, created a video for MozCamp Mumbai. It’s about 20 minutes long and he covers a huge number of topics: the new @font-face CSS property and how it affects the ability for people to receive properly localized content, the differences between the various standards efforts (there’s more than just HTML5) and gives some demos that show what’s possible when you combine video with the web.

    Download: 640×480 – Ogg Theora or MP4 | 320×240 – Ogg Theora or MP4
  7. HTML5 drag and drop in Firefox 3.5

    This post is from Les Orchard, who works on Mozilla’s web development team.


    Drag and drop is one of the most fundamental interactions afforded by graphical user interfaces. In one gesture, it allows users to pair the selection of an object with the execution of an action, often including a second object in the operation. It’s a simple yet powerful UI concept used to support copying, list reordering, deletion (ala the Trash / Recycle Bin), and even the creation of link relationships.

    Since it’s so fundamental, offering drag and drop in web applications has been a no-brainer ever since browsers first offered mouse events in DHTML. But, although mousedown, mousemove, and mouseup made it possible, the implementation has been limited to the bounds of the browser window. Additionally, since these events refer only to the object being dragged, there’s a challenge to find the subject of the drop when the interaction is completed.

    Of course, that doesn’t prevent most modern JavaScript frameworks from abstracting away most of the problems and throwing in some flourishes while they’re at it. But, wouldn’t it be nice if browsers offered first-class support for drag and drop, and maybe even extended it beyond the window sandbox?

    As it turns out, this very wish is answered by the HTML 5 specification section on new drag-and-drop events, and Firefox 3.5 includes an implementation of those events.

    If you want to jump straight to the code, I’ve put together some simple demos of the new events.

    I’ve even scratched an itch of my own and built the beginnings of an outline editor, where every draggable element is also a drop target—of which there could be dozens to hundreds in a complex document, something that gave me some minor hair-tearing moments in the past while trying to make do with plain old mouse events.

    And, all the above can be downloaded or cloned from a GitHub repository I’ve created expecially for this article.

    The New Drag and Drop Events

    So, with no further ado, here are the new drag and drop events, in roughly the order you might expect to see them fired:

    A drag has been initiated, with the dragged element as the event target.
    The mouse has moved, with the dragged element as the event target.
    The dragged element has been moved into a drop listener, with the drop listener element as the event target.
    The dragged element has been moved over a drop listener, with the drop listener element as the event target. Since the default behavior is to cancel drops, returning false or calling preventDefault() in the event handler indicates that a drop is allowed here.
    The dragged element has been moved out of a drop listener, with the drop listener element as the event target.
    The dragged element has been successfully dropped on a drop listener, with the drop listener element as the event target.
    A drag has been ended, successfully or not, with the dragged element as the event target.

    Like the mouse events of yore, listeners can be attached to elements using addEventListener() directly or by way of your favorite JS library.

    Consider the following example using jQuery, also available as a live demo:

        <div id="newschool">
            <div class="dragme">Drag me!</div>
            <div class="drophere">Drop here!</div>
        <script type="text/javascript">
            $(document).ready(function() {
                $('#newschool .dragme')
                    .attr('draggable', 'true')
                    .bind('dragstart', function(ev) {
                        var dt = ev.originalEvent.dataTransfer;
                        dt.setData("Text", "Dropped in zone!");
                        return true;
                    .bind('dragend', function(ev) {
                        return false;
                $('#newschool .drophere')
                    .bind('dragenter', function(ev) {
                        return false;
                    .bind('dragleave', function(ev) {
                        return false;
                    .bind('dragover', function(ev) {
                        return false;
                    .bind('drop', function(ev) {
                        var dt = ev.originalEvent.dataTransfer;
                        return false;

    Thanks to the new events and jQuery, this example is both short and simple—but it packs in a lot of functionality, as the rest of this article will explain.

    Before moving on, there are at least three things about the above code that are worth mentioning:

    • Drop targets are enabled by virtue of having listeners for drop events. But, per the HTML 5 spec, draggable elements need an attribute of draggable="true", set either in markup or in JavaScript.

      Thus, $('#newschool .dragme').attr('draggable', 'true').

    • The original DOM event (as opposed to jQuery’s event wrapper) offers a property called dataTransfer. Beyond just manipulating elements, the new drag and drop events accomodate the transmission of user-definable data during the course of the interaction.
    • Since these are first-class events, you can apply the technique of Event Delegation.

      What’s that? Well, imagine you have a list of 1000 list items—as part of a deeply-nested outline document, for instance. Rather than needing to attach listeners or otherwise fiddle with all 1000 items, simply attach a listener to the parent node (eg. the <ul> element) and all events from the children will propagate up to the single parent listener. As a bonus, all new child elements added after page load will enjoy the same benefits.

      Check out this demo, and the associated JS code to see more about these events and Event Delegation.

    Using dataTransfer

    As mentioned in the last section, the new drag and drop events let you send data along with a dragged element. But, it’s even better than that: Your drop targets can receive data transferred by content objects dragged into the window from other browser windows, and even other applications.

    Since the example is a bit longer, check out the live demo and associated code to get an idea of what’s possible with dataTransfer.

    In a nutshell, the stars of this show are the setData() and getData() methods of the dataTransfer property exposed by the Event object.

    The setData() method is typically called in the dragstart listener, loading dataTransfer up with one or more strings of content with associated recommended content types.

    For illustration, here’s a quick snippet from the example code:

        var dt = ev.originalEvent.dataTransfer;
        dt.setData('text/plain', $('#logo').parent().text());
        dt.setData('text/html', $('#logo').parent().html());
        dt.setData('text/uri-list', $('#logo')[0].src);

    On the other end, getData() allows you to query for content by type (eg. text/html followed by text/plain). This, in turn, allows you to decide on acceptable content types at the time of the drop event or even during dragover to offer feedback for unacceptable types during the drag.

    Here’s another example from the receiving end of the example code:

        var dt = ev.originalEvent.dataTransfer;
        $('.content_url .content').text(dt.getData('text/uri-list'));
        $('.content_text .content').text(dt.getData('text/plain'));
        $('.content_html .content').html(dt.getData('text/html'));

    Where dataTransfer really shines, though, is that it allows your drop targets to receive content from sources outside your defined draggable elements and even from outside the browser altogether. Firefox accepts such drags, and attempts to populate dataTransfer with appropriate content types extracted from the external object.

    Thus, you could select some text in a word processor window and drop it into one of your elements, and at least expect to find it available as text/plain content.

    You can also select content in another browser window, and expect to see text/html appear in your events. Check out the outline editing demo and see what happens when you try dragging various elements (eg. images, tables, and lists) and highlighted content from other windows onto the items there.

    Using Drag Feedback Images

    An important aspect of the drag and drop interaction is a representation of the thing being dragged. By default in Firefox, this is a “ghost” image of the dragged element itself. But, the dataTransfer property of the original Event object exposes the method setDragImage() for use in customizing this representation.

    There’s a live demo of this feature, as well as associated JS code available. The gist, however, is sketched out in these code snippets:

        var dt = ev.originalEvent.dataTransfer;
        dt.setDragImage( $('#feedback_image h2')[0], 0, 0);
        dt.setDragImage( $('#logo')[0], 32, 32);
        var canvas = document.createElement("canvas");
        canvas.width = canvas.height = 50;
        var ctx = canvas.getContext("2d");
        ctx.lineWidth = 8;
        ctx.lineTo(50, 50);
        ctx.lineTo(0, 50);
        ctx.lineTo(25, 0);
        dt.setDragImage(canvas, 25, 25);

    You can supply a DOM node as the first parameter to setDragImage(), which includes everything from text to images to <canvas> elements. The second two parameters indicate at what left and top offset the mouse should appear in the image while dragging.

    For example, since the #logo image is 64×64, the parameters in the second setDragImage() method places the mouse right in the center of the image. On the other hand, the first call positions the feedback image such that the mouse rests in the upper left corner.

    Using Drop Effects

    As mentioned at the start of this article, the drag and drop interaction has been used to support actions such as copying, moving, and linking. Accordingly, the HTML 5 specification accomodates these operations in the form of the effectAllowed and dropEffect properties exposed by the Event object.

    For a quick fix, check out the a live demo of this feature, as well as the associated JS code.

    The basic idea is that the dragstart event listener can set a value for effectAllowed like so:

        var dt = ev.originalEvent.dataTransfer;
        switch ( {
            case 'effectdrag0': dt.effectAllowed = 'copy'; break;
            case 'effectdrag1': dt.effectAllowed = 'move'; break;
            case 'effectdrag2': dt.effectAllowed = 'link'; break;
            case 'effectdrag3': dt.effectAllowed = 'all'; break;
            case 'effectdrag4': dt.effectAllowed = 'none'; break;

    The choices available for this property include the following:

    no operation is permitted
    copy only
    move only
    link only
    copy or move only
    copy or link only
    link or move only
    copy, move, or link

    On the other end, the dragover event listener can set the value of the dropEffect property to indicate the expected effect invoked on a successful drop. If the value does not match up with effectAllowed, the drop will be considered cancelled on completion.

    In the a live demo, you should be able to see that only elements with matching effects can be dropped into the appropriate drop zones. This is accomplished with code like the follwoing:

        var dt = ev.originalEvent.dataTransfer;
        switch ( {
            case 'effectdrop0': dt.dropEffect = 'copy'; break;
            case 'effectdrop1': dt.dropEffect = 'move'; break;
            case 'effectdrop2': dt.dropEffect = 'link'; break;
            case 'effectdrop3': dt.dropEffect = 'all'; break;
            case 'effectdrop4': dt.dropEffect = 'none'; break;

    Although the OS itself can provide some feedback, you can also use these properties to update your own visible feedback, both on the dragged element and on the drop zone itself.


    The new first-class drag and drop events in HTML5 and Firefox make supporting this form of UI interaction simple, concise, and powerful in the browser. But beyond the new simplicity of these events, the ability to transfer content between applications opens brand new avenues for web-based applications and collaboration with desktop software in general.

  8. css transforms: styling the web in two dimensions

    One feature that Firefox 3.5 adds to its CSS implementation is transform functions. These let you manipulate elements in two dimensional space by rotating, skewing, scaling, and translating them to alter their appearance.

    I’ve put together a demo that shows how some of these functions work.

    There are four animating objects in this demo. Let’s take a look at each of them.

    Rotating the Firefox logo

    On the left, we see the Firefox logo in a nice box, happily spinning in place. This is done by periodically setting the rotation value of the image object, whose ID is logoimg, like this:

      var logo = document.getElementById("logoimg");
      logoAngle = logoAngle + 2;
      if (logoAngle >= 360) {
        logoAngle = logoAngle - 360;
      var style = "-moz-transform: rotate(" + logoAngle + "deg)";
      logo.setAttribute("style", style);

    Every time the animation function is run, we rotate it by 2° around its origin by constructing a style string of the form -moz-transform: rotate(Ndeg).

    By default, all elements’ origins are at their centers (that is, 50% along each axis). The origin can be changed using the -moz-transform-origin attribute.

    Skewing text

    We have two examples of skewing in this demo; the first skews horizontally, which causes the text to “lean” back and forth along the X axis. The second skews vertically, which causes the baseline to pivot along the Y axis.

    In both cases, the code to accomplish this animation is essentially identical, so let’s just look at the code for skewing horizontally:

      text1SkewAngle = text1SkewAngle + text1SkewOffset;
      if (text1SkewAngle > 45) {
        text1SkewAngle = 45;
        text1SkewOffset = -2;
      } else if (text1SkewAngle < -45) {
        text1SkewAngle = -45;
        text1SkewOffset = 2;
   = "skewx(" + text1SkewAngle + "deg)";

    This code updates the current amount by which the text is skewed, starting at zero degrees and moving back and forth between -45° and 45° at a rate of 2° each time the animation function is called. Positive values skew the element to the right and negative values to the left.

    Then the element’s transform style is updated, setting the transform function to be of the form skewx(Ndeg), then setting the element’s style.MozTransform property to that value.

    Scaling elements

    The last of the examples included in the demo shows how to scale an element using the scale transform function:

      text3Scale = text3Scale + text3ScaleOffset;
      if (text3Scale > 6) {
        text3Scale = 6;
        text3ScaleOffset = -0.1;
        text3.innerHTML = "It's going away so fast!" = "blue";
      } else if (text3Scale < 1) {
        text3Scale = 1;
        text3ScaleOffset = 0.1;
        text3.innerHTML = "It's coming right at us!"; = "red";
   = "scale(" + text3Scale + ")";

    This code scales the element up and down between its original size (a scale factor of 1) and a scale factor of 6, moving by 0.1 units each frame. This is done by building a transform of the form scale(N), then setting the element’s style.MozTransform property to that value.

    In addition, just for fun, we’re also changing the text and the color of the text in the block as we switch scaling directions, by setting the value of the block’s innerHTML property to the new contents.

    Final notes

    Three more tidbits to take away from this:

    First, note that as the scaling text grows wider, the document’s width changes to fit it, getting wider as the text grows so that its right edge passes the edge of the document, then narrower as it shrinks again. You can see this by watching the scroll bar at the bottom of the Firefox browser window.

    Second, note that you can actually select and copy the text not only while the elements are transformed, but the selection remains intact while the text continues to transform (although when we change the contents of the scaling example, the selection goes away).

    Third, I didn’t cover all the possible transforms here. For example, I skipped over the translate transform function, which lets you translate an object horizontally or vertically (basically, shifting its position by an offset). You can get a full list of the supported transforms on the Mozilla Developer Center web site.

    Obviously this demo is somewhat frivolous (as demos are prone to be). However, there are genuinely useful things you can do with these when designing interfaces; for example, you can draw text rotated by 90° along the Y axis of a table in order to fit row labels in a narrow but tall space.

  9. using web workers: working smarter, not harder

    This article is written by Malte Ubl, who has done a lot of great work with using Web Workers as part of the bespin project.

    In recent years, the user experience of web applications has grown richer and richer. In-browser applications like GMail, Meebo and Bespin give us an impression of how the web will look and feel in the future. One of the key aspects of creating a great user experience is to build applications that are highly responsive. Users hate to wait and they hate those moments where an application seems to work for a while, then stops responding to their input.

    At the core of modern client-side web applications lies the JavaScript programming language. JavaScript and the DOM that it talks to is inherently single-threaded. This means that in JavaScript only one thing can happen at any given time. Even if your computer has 32 cores it will keep only one of those cores busy when it’s doing a long computation.  For example if you calculate the perfect trajectory to get to the moon it won’t be able to render an animation that shows the trajectory at the same time and it won’t be able to react to any user events like clicks or typing on the keyboard while it’s doing that calculation.


    To maintain responsiveness while performing intense computations concurrency is a part of most modern programming languages. In the past concurrency was often achieved by the use of threads. Threads, however, make it increasingly hard for the programmer to understand the program flow which often leads to very hard to understand bugs and chaotic behavior when different threads manipulate the same data simultaneously.

    Web Workers, which were recommended by the WHATWG, were introduced in Firefox 3.5 to add concurrency to JavaScript applications without also introducing the problems associated with multithreaded programs. Starting a worker is easy – just use the new Worker interface.

    In this example the worker.js file will be loaded and the a new thread will be created to execute that code.

    // Start worker from file "worker.js"
    var worker = new Worker("worker.js");

    Communication between the main UI thread and workers is done by passing messages using the postMessage method. postMessage was added for cross-window communication in Firefox 3. To send a message from the worker back to the page, you just post a message:

    // Send a message back to the main UI thread
    postMessage("Hello Page!");

    To catch the message from the worker, you define an “onmessage” callback on the worker object. Here we just alert the event data that is passed to the callback function. In this case, “” contains the “Hello Page!” string that was sent above.

    worker.onmessage = function (event) {
      // Send a message to the worker
      worker.postMessage("Hello Worker");

    To send a message to the worker we call the postMessage method on the worker object. To receive these messages inside the worker, simply define an onmessage function that will be called every time a message is posted to the worker.

    Error Handling

    There are two levels at which you can recover from runtime errors that occur in a worker. First, you can define an onerror function within the worker. Second, you can handle errors from the outside the worker by attaching an onerror handler on to the worker object:

    worker.onerror = function (event) {

    The event.preventDefault() method prevents the default action, which would be to display the error to the user or at least show it in the error console. Here we alert the error message instead.

    Shared Nothing

    Workers share absolutely no state with the page they are associated with or with any other workers; the only way they can interact at all is through postMessage. Workers also have no access to the DOM, so they can not directly manipulate the web page. There is thus no risk of problems with data integrity when multiple workers want to manipulate the same data at once.

    A standard setup that is using workers would consist of a page JavaScript component that is listening for user events. When an event occurs that triggers an intensive calculation a message is sent to the worker which then starts the computation. The script on the page, however, can terminate immediately and listen for more user events. As soon as the worker is done, it sends a return message to the page which can then, for example, display the result.

    The unresponsive script warning that is being displayed by browsers when a script is taking a long time to execute is a thing of the past when using web workers.

    The Fibonacci Example

    Next is an example of a worker that calculates the Fibonacci numbers from 0 to 99 in the background. Actually, because calculating Fibonacci numbers using this very inefficient method can take a lot of time for larger numbers (as in greater than something like 30) the script might never finish on your computer (or crash because it blows out the stack), but when doing it in a worker this has no effect on the responsiveness of the main web page. So you can still draw a complex animation to make the waiting time for the next number a little more fun.

    This HTML page contains a script that starts a worker from the file “fib-worker.js”. Messages from the worker are displayed on the browser’s console using console.log.

    <!DOCTYPE html>
          <title>Web Worker API Demo</title>
          <script type="text/javascript">
            var worker = new Worker("fib-worker.js");
            worker.onmessage = function (event) {
              console.log( +" -> " +

    The JavaScript file that implements the worker contains a loop that calculates Fibonacci numbers and sends the result to the page.

    // File fib-worker.js
    function fib(n) {
       return n < 2 ? n : fib(n-1) + fib(n-2);
    for(var i = 0; i < 100; ++i) {
          index: i,
          value: fib(i)

    In the example above we see that we can also pass complex objects to the postMessage. These objects can contain everything that can be transmitted via JSON. This means that functions cannot be passed across worker boundaries and that the objects are passed by value rather than by reference.

    Worker APIs

    Workers support a function called importScripts. You can use this to load more source files into the worker.

    importScripts("foo.js", "bar.js");

    When you pass multiple parameters to the function the scripts will be downloaded in parallel but executed in the order of definition. The function does not return until all scripts have been downloaded and executed.

    Here we load an external JavaScript file that calculates SHA-1 hash sums from strings and then we use it to hash responses from AJAX requests. We also use the standard XMLHttpRequest object to retrieve the content of the URL which is passed in via the onmessage event. The interesting part is that we don’t have to worry about making the AJAX request asynchronous because the worker itself is asynchronous with respect to page rendering, so a little waiting for the HTTP request does not hurt as much.

    function onmessage(event) {
        var xhr = new XMLHttpRequest();'GET',, false);

    Other APIs Available to Workers

    Workers may use XMLHttpRequest for AJAX requests as seen above and access the client sided database using web storage API. Here the APIs are identical to their usage in regular JavaScript.

    The setTimeout and setInterval (and the clearTimeout and clearInterval friends) functions, which enable executing code after a given period of time or at certain intervals, are also available within the worker as is the well known navigator object, which can be inspected to get information about the current browser.

    More APIs may be added in the future.

    Browser Compatibility

    As of this writing (and to the knowledge of the author), Firefox 3.5 is the only browser that supports the ability to pass complex objects via postMessage and that implements the extended APIs defined above. Safari 4 implements a very basic version of the Worker API. For other browsers it is possible to use Workers via Google Gears, which originally introduced the concept to browsers.

    Real World Usage

    In the Bespin project, which is a browser based source code editor, we successfully used workers to implement CPU intensive features like real-time source code error checking and code completion. We also created a shim that implements the Worker API in terms of Google Gears and which adds the missing features to the worker implementation of Safari 4 and also moved to using transparent custom events on top of the postMessage interface. These components will be released as a stand-alone library to be usable in other projects in the future.

    Web Workers will play an important role in making the Open Web an even more powerful platform for sophisticated applications. Because in the end all they do is execute JavaScript, it’s easy to make scripts work on clients which do yet have the luxury of web workers. So go ahead and add them to your applications today to make them feel just a little more responsive and more pleasant to use.

  10. exploring music with the audio tag

    Today’s demo comes to us from Samuel Goldszmidt. He’s a web developer specializing in audio applications at Institut de Recherche et Coordination Acoustique/Musique (IRCAM). IRCAM is a European institute covering science, sound and avant garde electro-acoustical art music.

    The demo uses XML to describe the various segments of a piece of music – Florence Baschet’s StreicherKreis (Circle of Strings). The music itself is a combination of stringed instruments and electronic effects. From the XML, SVG is generated for each section of the music. You can click on each section to listen to that part of the piece and a description is shown on how that particular section was created.

    As far as demos go, this is relatively simple. But it’s worth highlighting because it shows how easy it is to build a timeline around a piece of music and add descriptive information. In this case, it’s information meant to teach people how a particular effect was created. But it could be anything, from showing different camera angles of people playing the music to links about different covers of a popular piece. Opening up media to the web means that we can combine it with text, images and other media. This is just a small example.