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This Week in HTML 5 – Episode 10

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.


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:

Around the web:

Tune in next week for another exciting episode of "This Week in HTML 5."

Posted in Weekly Review | 5 Comments »

This Week in HTML 5 – Episode 6

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 »