Tuesday, November 18th, 2008
Welcome back to "This Week in HTML 5," where I'll try to summarize the major activity in the ongoing standards process in the WHATWG and W3C HTML Working Group.
The big news this week is a major revamping of how browsers should process multimedia in the <audio> and <video> elements.
r2404 makes a number of important changes. First, the canPlayType() method has moved from the navigator object to HTMLMediaElement (i.e. a specific <audio> or <video> element), and it now returns a string rather than an integer. [canPlayType() discussion]
The canPlayType(type) method must return the string "no" if type is a type that the user agent knows it cannot render; it must return "probably" if the user agent is confident that the type represents a media resource that it can render if used in with this audio or video element; and it must return "maybe" otherwise. Implementors are encouraged to return "maybe" unless the type can be confidently established as being supported or not. Generally, a user agent should never return "probably" if the type doesn't have a codecs parameter.
Wait, what codecs parameter? That's the second major change: the <source type> attribute (which previously could only contain a MIME type like "video/mp4", which is insufficient to determine playability) can now contain a MIME type and a codecs parameter. As specified in RFC 4281, the codecs parameter specifies the specific codecs used by the individual streams within the audio/video container. The section on the type attribute contains several examples of using the codecs parameter.
Third, the <source type> attribute is now optional. If you aren't sure what kind of video you're serving, you can just throw one or more <source> elements into a <video> element and the browser will try each of them in the order specified [r2403] until it finds something it can play. [load() algorithm discussion] Of course, if you include a type attribute (and codecs parameter within it), the browser may use it to determine playability without loading multiple resources, but this is no longer required.
The final change (this week) to multimedia elements is the elimination of the start, end, loopstart, loopend, and playcount attributes. They are all replaced by a single attribute, loop, which takes a boolean. To handle initially seeking to a specific timecode (like the now-eliminated start attribute), the HTML 5 spec vaguely declares, "For example, a fragment identifier could be used to indicate a start position." This obviously needs further specification.
One multimedia-related issue that did not change in the spec this week is same-origin checking for media elements. Robert O'Callahan asked whether video should be allowed to load from another domain, noting (correctly) that it could lead to information leakage about files posted on private intranets. Chris Double outlines the issues and some proposed solutions. However, contrary to Chris' expectation, HTML 5 will not (yet) mandate cross-site restrictions for audio/video files. This is an ongoing discussion. [WHATWG discussion thread, Theora discussion thread]
In other news, Ian Hickson summarized the discussion around the <input placeholder> attribute (which I first mentioned in This Week in HTML 5 Episode 8) and committed r2409 that defines the new attribute:
The placeholder attribute represents a short hint (a word or short phrase) intended to aid the user with data entry. A hint could be a sample value or a brief description of the expected format.
For a longer hint or other advisory text, the title attribute is more appropriate.
The placeholder attribute should not be used as an alternative to a label.
User agents should present this hint to the user only when the element's value is the empty string and the control is not focused (e.g. by displaying it inside a blank unfocused control).
Read the section on the placeholder attribute for an example of its proper use.
Other interesting tidbits this week:
Around the web:
- The W3C published an editor's draft of Lachlan Hunt's Web Developer's Guide to HTML 5.
- Austin Chau posted a demo of HTML 5 cross-document messaging. Further discussion: Using HTML5 postMessage, postMessage API changes, and the unfortunately-named xssinterface library which implements a
postMessage-like API in browsers that do not yet support it.
- Ryan Tomayko posted an excellent summary of things caches do, specifically HTTP caches like Squid and rack-cache.
- Joe Clark posted This is How the Web Gets Regulated, a call to action on video accessibility.
- mv_embed is a GPL-licensed Javascript shim that takes
<video> elements that point to Ogg Theora video files and replaces them with plugin-specific markup to play the video through oggplay, vlc-plugin, Java cortado, mplayer, Totem, or Apple Quicktime (if Xiph's Ogg Theora Quicktime component is installed). A demo page demonstrates the technique.
- Everyone should go admire my new dog Beauregard, then scroll down to read "dave"'s non-Beau-related but extremely interesting comment on an experimental Ogg Theora video encoder. From there, I learned about this Ogg Vorbis audio decoder written in pure ActionScript (Flash), leading to the tantalizing but as-yet-unrealized possibility of a Javascript shim like mv_embed that could take
<audio> elements that point to Ogg Vorbis audio files and replace them with a Flash wrapper that could play the audio file, even in browsers that do not support the <audio> element or the Ogg Vorbis audio codec.
Tune in next week for another exciting episode of "This Week in HTML 5."
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Monday, October 20th, 2008
Welcome back to "This Week in HTML 5," where I'll try to summarize the major activity in the ongoing standards process in the WHATWG and W3C HTML Working Group.
The big news this week is offline caching. This has been in HTML 5 for a while, but this week Ian Hickson caught up with his email and integrated all outstanding feedback. He summarizes the changes:
- Made the online whitelist be prefix-based instead of exact match. [r2337]
- Removed opportunistic caching, leaving only the fallback behavior part. [r2338]
- Made fallback URLs be prefix-based instead of only path-prefix based (we no longer ignore the query component). [r2343]
- Made application caches scoped to their browsing context, and allowed iframes to start new scopes. By default the contents of an iframe are part of the appcache of the parent, but if you declare a manifest, you get your own cache. [r2344]
- Made fallback pages have to be same-origin (security fix). [r2342]
- Made the whole model treat redirects as errors to be more resilient in the face of captive portals when offline (it's unclear what else would actually be useful and safe behavior anyway). [r2339]
- Fixed a bunch of race conditions by redefining how application caches are created in the first place. [r2346]
- Made 404 and 410 responses for application caches blow away the application cache. [r2348]
- Made checking and downloading events fire on ApplicationCache objects that join an update process midway. [r2353]
- Made the update algorithm check the manifest at the start and at the end and fail if the manifest changed in any way. [r2350]
- Made errors on master and dynamic entries in the cache get handled in a non-fatal manner (and made 404 and 410 remove the entry). [r2348]
- Changed the API from .length and .item() to .items and .hasItem(). [r2352]
And now, a short digression into video formats...
You may think of video files as "AVI files" or "MP4 files". In reality, "AVI" and "MP4" are just container formats. Just like a ZIP file can contain any sort of file within it, video container formats only define how to store things within them, not what kinds of data are stored. (It's a little more complicated than that, because container formats do limit what codecs you can store in them, but never mind.) A video file usually contains multiple tracks -- a video track (without audio), one or more audio tracks (without video), one or more subtitle/caption tracks, and so forth. Tracks are usually inter-related; an audio track contains markers within it to help synchronize the audio with the video, and a subtitle track contains time codes marking when each phrase should be displayed. Individual tracks can have metadata, such as the aspect ratio of a video track, or the language of an audio or subtitle track. Containers can also have metadata, such as the title of the video itself, cover art for the video, episode numbers (for television shows), and so on.
Individual video tracks are encoded with a certain video codec, which is the algorithm by which the video was authored and compressed. Modern video codecs include H.264, DivX, VC-1, but there are many, many others. Audio tracks are also encoded in a specific codec, such as MP3, AAC, or Ogg Vorbis. Common video containers are ASF, MP4, and AVI. Thus, saying that you have sent someone an "MP4 file" is not specific enough for the recipient to determine if they can play it. The recipient needs to know the container format (such as MP4 or AVI), but also the video codec (such as H.264 or Ogg Theora) and the audio codec (such as MP3 or Ogg Vorbis). Furthermore, video codecs (and some audio codecs) are broad standards with multiple profiles, so saying that you have sent someone an "MP4 file with H.264 video and AAC audio" is still not specific enough. An iPhone can play MP4 files with "baseline profile" H.264 video and "low complexity" AAC audio. (These are well-defined technical terms, not laymen's terms.) Desktop Macs can play MP4 files with "main profile" H.264 video and "main profile" AAC audio. Adobe Flash can play MP4 files with "high profile" H.264 video and "HE" AAC audio. Of course, it's a little more complicated than that.
Thus...
r2332 adds a navigator.canPlayType() method. This is intended for scripts to query whether the client can play a certain type of video. There are two major problems with this: first, MIME types are not specific enough, as they will only describe the video container. Learning that the client "can play" MP4 files is useless without knowing what video codecs it supports inside the container, not to mention what profiles of that video codec it supports. The second problem is that, unless the browser itself ships with support for specific video and audio codecs (as Firefox 3.1 will do with Ogg Theora and Ogg Vorbis), it will need to rely on some multimedia library provided by the underlying operating system. Windows has DirectShow, Mac OS X has QuickTime, but neither of these libraries can actually tell you whether a codec is supported. The best you can do is try to play the video and notice if it fails. [WHATWG thread]
Other interesting changes and discussions this week:
r2333 changes the data type of the width and height attributes on <embed>, <object>, and <iframe> elements to match current browser behavior. These attributes reflect strings, not integers. No one knows why.
Ian Hickson kicked off another round of video accessibility discussion, with this philosophical statement:
Fundamentally, I consider <video> and <audio> to be simply windows onto pre-existing content, much like <iframe>, but for media data instead of for "pages" or document data. Just as with <iframe>s, the principle I had in mind is that it should make sense for the user to take the content of the element and view it independent of its hosting page. You should be able to save the remote file locally and open it in a media player and you should be able to write a new page with a different media player interface, without losing any key aspect of the media. In particular, any accessibility features must not be lost when doing this. For example, if the video has subtitles or PiP hand language signing, or multiple audio tracks, or a transcript, or lyrics, or metadata, _all_ of this data should survive even if the video file is saved locally without the embedding page.
In other words, video accessibility should be handled within the video container, not in the surrounding HTML markup. On the plus side, all modern video containers can handle subtitle tracks, secondary audio tracks, and so forth. Unfortunately, authors may be hesitant to add to their bandwidth costs by including tracks that must be downloaded by everyone but appreciated (or even noticed) by very few.
[W3C discussion thread on video accessibility]
Sander van Zoest noticed the pixelaspectratio attribute of the <video> element, and he asked why it was a float instead of a ratio of two rationals (as is standard practice in the video authoring world). Ultimately, he agreed with Eric Carlson that pixelaspectratio should be dropped from HTML 5 because it doesn't really give enough information about how to scale the video properly. As with so many other things in the video world, the problem is much more complicated that it first appears.
Around the web:
Tune in next week for another exciting episode of "This Week in HTML 5."
Posted in Weekly Review | 5 Comments »
Tuesday, September 23rd, 2008
Welcome back to "This Week in HTML 5," where I'll try to summarize the major activity in the ongoing standards process in the WHATWG and W3C HTML Working Group.
There is no big news this week. Work continued on last week's orgy of Web Forms-related check-ins. This week adds the <label> element and the jack-of-all-forms <input> element. [r2191, r2192, r2197, r2200, r2202, r2204, r2205, r2207, r2211, r2212, r2213, r2214, r2218, r2219, r2220, r2222, r2223]
Laura Carlson and others have begun to review the accessibility of multimedia on the web. Most accessibility discussions revolve around the needs of visually impaired users, but hearing impaired users are also important and too often ignored. There was a long discussion last month (and continuing into this month) about the accessibility implications of the <audio> and <video> elements for hearing impaired users. YouTube (owned by Google, my employer) recently announced support for captions on YouTube videos and published a tutorial on adding them to your own videos.
Ian Hickson (the HTML 5 editor) gave an interview about HTML 5 in which he reiterated his goal of having two independent, complete, interoperable implementations of HTML 5 by 2022. (By contrast, HTML 4.0 was "finalized" 11 years ago but still doesn't have two independent, complete, interoperable implementations.) This led to a mini-firestorm among bloggers who misunderstood "2022" as "the date when I can start using HTML 5 features." It bears repeating that the "2022" date has no significance at all for web developers. Most browser vendors are actively involved in HTML 5, several browsers are already shipping HTML 5 features, and developers who are holding their breath until 2022 are going to find themselves seriously behind the curve.
On that note, Brenton Strine asks a very good question: "Is there some place that documents the parts of HTML 5 that are already up and running? Can I use <canvas> or <video>? In which browsers? What other tags can I use? What other fancy HTML 5 stuff can I do today in 2008?" On the video front, Mozilla will be shipping Ogg Theora support in Firefox 3.1. (You can read more about why Ogg matters.) Last year, Opera released experimental builds with Ogg Theora support, and they now have video-enabled builds on 3 platforms. The Wikimedia Foundation has a few Theora-encoded videos you can watch.
Tune in next week for another exciting episode of "This Week in HTML 5."
Posted in Weekly Review | 6 Comments »