[whatwg] Markup for external content
alexey at feldgendler.ru
Sun Mar 18 10:53:39 PDT 2007
Hope it's not too late to add my opinion to the discussion about <video>.
This posting expresses my view on the various kinds of markup for external
Under external content I understand informative content presented
out-of-line of the HTML document and referenced from the latter by means
of a URI. Also, the equivalent of referencing external content with an URI
can be embedding some content expressed in an XML-based language other
than XHTML into an XHMTL document (SVG, MathML) -- this case should be
treated the same as the case of true out-of-line content.
There is a set of types which the user agent is capable of handling
(either internally or with help of external software, as long as it's done
automatically). In the most vulgar example of a traditional GUI browser,
if you enter an URL of a resource having one of these types into the
address bar, the browser will somehow present the content to the user
(render the hypertext document, display the image, play the video or audio
clip, etc). Generally, external content can be of any of these types.
External content can have or not have certain properties, which are not
* Replaced: under replaced content I understand something that is
presented in a rectangular area of a visual medium. Images and videos are
examples of replaced content, though it's not an exhaustive list.
* Timed: such content that is presented as time goes. Examples are video
and audio clips. Typically, such operations as "play", "pause", "seek"
apply to them, though not necessarily (e.g. a live TV broadcast usually
cannot be sought in).
* Interactive: implements its own handling of user input (clicks,
keypresses) not expressed by HTML markup. Flash controls are examples.
* Structured: the content is a document whose structure is represented in
a DOM. Examples are HTML documents, SVG and MathML resources.
Basing on these properties, it's possible to define some generic content
types (not to be confused with data formats):
* Image: replaced.
* Audio: timed, NOT replaced.
* Video: replaced, timed.
* Subdocument: replaced, structured.
* Control: replaced, interactive.
Note that "video", "subdocument", and "control" are a subset of "image"
(and they all effectively degrade to "image" when rendered on a
non-interactive medium such as paper).
[X]HTML has, or is going to have, several elements to support external
* <object>: any external content. This is the most basic and meaningless
markup for external content, similar to <div> being the most basic and
meaningless markup for hypertext. <object> places no restrictions on the
properties of the content. In addition to being semantically meaningless,
<object> also has historical interoperability problems (as in "always was
* <img>: image. Note that because "image" is a superset of "video". And
indeed, <img> has been supporting one video format for decades: animated
GIF. I can see no reason why <img> shouldn't support e.g. Ogg Theora in
user agents which support this format; however, for authors it would be
more desirable to use <video> for this purpose (and, probably, for
animated GIF, too). Likewise, I see no reason why <img> shouldn't support
SVG or Flash (alright, there are security issues with Flash...). Even
though <object> can also display images, <img> is preferable because it's
specifically designed for images and provides image-specific features such
as scaling to a fixed width and height while maintaining the aspect ratio,
and possibly some visual controls such as IE's image toolbar. <img> is
also more semantically precise.
* <video>: video. Should support any video format which can be decoded
either by the UA internally or by plugins, external programs etc as long
as it's automatic. Should also support animated GIF and SVG because these
are essentially video. Even though <img> and even <object> can also play
video, the <video> element is designed specifically for video and provides
video-specific features: scaling logic most useful for video clips, a DOM
API, and possibly some visual playback controls. <video> is also more
* <audio> and <bgsound>: audio. Should support any audio format which can
be decoded either by the UA internally or by plugins, external programs
etc as long as it's automatic. Similar benefits over <object> as above.
I'm not sure what the relation between <audio> and <bgsound> is going to
be. Probably <audio> should act like replaced content by rendering some
* <embed>: the old way of activating plugins; references any external
content for which a plugin is used. I believe that HTML should be
independent of the implementations architecture, so knowledge of whether
or not a plugin is needed to render a specific format should not be
encoded in HTML documents. <embed> must die.
* <applet>: the (old) way of activating Java. Probably must also die,
though I'm unsure about this one.
* <iframe>: subdocument. Though <object> can do the same, <iframe> is
specifically designed for structured content and provides a DOM API for
access to the contained DOM. Also more semantically precise.
* Foregin namespaces in XHTML (<svg xmlns="http://www.w3.org/2000/svg">
and such): subdocument. This method allows access to the contained DOM but
doesn't expose any image-, audio-, or video-specific API. Probably it
About plugins: in early browser development, it used to be the case that
every new browser feature got itself a new element. That's how come we now
have <object>, <embed>, and <applet>. So, <embed> was the way to mark up
certain types of external content just because the first browser to
implement it did so with a plugin. I believe such implementation details
should not affect the design of HTML anymore. There shouldn't be any
specific markup for "plugin content" because we never know whether support
for a particular format is going to be implemented with a plugin. There
are plugins which add PNG support to old browsers. On the other hand, it's
possible to implement Flash support natively using an open-source player
library. Especially on a mobile device it's unlikely that the browser is
going to have any plugin system.
That's why I strongly disagree with the idea of having <video> only
support open formats and leaving proprietary formats to <object>. Any
video format which the browser can play, no matter natively or through a
plugin, should be supported with <video>, as long as it's technically
possible to expose a woring <video> DOM API to control the underlying
implementation. Even Flash supports the notion of pausing and seeking to
some extent, and is therefore a candidate for <video>.
Alexey Feldgendler <alexey at feldgendler.ru>
[ICQ: 115226275] http://feldgendler.livejournal.com
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