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Re: DRAFT analysis of fallback mechanisms for embedded content ACTION-66

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Thu, 25 Nov 2010 14:40:59 +1100
Message-ID: <AANLkTi=dryE2UN6HayCv5LhLT4P_qFVZq9Zt7uBWq7Lb@mail.gmail.com>
To: Michael Cooper <cooper@w3.org>
Cc: HTML Accessibility Task Force <public-html-a11y@w3.org>
Hi Michael,

I think this is tremendous work and should help us get a better structure
and consistent attribute names for accessibility.

I only have a few questions about the media elements.

For a short text alternative for video (not for audio though) I would expect
that what is being seen on screen as the representative image (be that a
poster or the first frame) needs some explanation, similar to the @alt on
img. I'd prefer that to label - what would the label be for?

Also, I am curious why you do not see a need for a summary on audio and
video? I can see that a full transcript goes into the long text alternative,
but a summary is different and useful to both, sighted and vision-impaired
users. In fact, it is often provided underneath the video or in an overlay.

And finally on caption (not captions but caption as used for images) - while
it's not customary to provide a caption subtext on video or audio, it is
very customary to display a video/audio title (and a short summary as
stated). Maybe in the video case the video "caption" is the video title? It
is also very important from an accessibility perspective since it explains
to sighted and thus also vision-impaired users what can be expected in the
video and should probably be made part of our concerns.

I'm not sure how the media use cases fit with your classification and it
would be great to clarify this.

Cheers,
Silvia.


On Thu, Nov 25, 2010 at 9:33 AM, Michael Cooper <cooper@w3.org> wrote:

>  The attached file is a draft analysis of how fallback mechanisms are
> applied to various types of embedded content in HTML. The analysis is an
> important step towards determining where the support provided by the HTML
> specification needs to be extended or modified to ensure all accessibility
> use cases are covered. The analysis is prepared primarily by Michael Cooper,
> with input from members of the Bug Triage sub-team of the HTML Accessibility
> Task Force.
>
> This is a draft analyis put forth for discussion and should not be viewed
> as complete, final, or constituting a proposal in itself.
>
> This analysis relates to Bug 8885<http://www.w3.org/Bugs/Public/show_bug.cgi?id=8885>and executes
> ACTION-66 <http://www.w3.org/WAI/PF/HTML/track/actions/66>. As discussed
> on 5 November 2010 <http://www.w3.org/2010/11/04-html-wg-minutes#item13>,
> preliminary work on this was done by the bug triage sub-team on 16
> November 2010 <http://www.w3.org/2010/11/16-a11y-bugs-minutes#item01> and 22
> November 2010 <http://www.w3.org/2010/11/23-a11y-bugs-minutes#item02>, and
> it is now being taken to the task force for wider review and discussion.
>
> Michael
> --
>
> Michael Cooper
> Web Accessibility Specialist
> World Wide Web Consortium, Web Accessibility Initiative
> E-mail cooper@w3.org <cooper@w3.org>
> Information Page <http://www.w3.org/People/cooper/>
>
>  Fallbacks for embedded content
>
> This is an attempt to analyze how fallback mechanisms are applied to
> various types of embedded content in HTML. The analysis is an important step
> towards determining where the support provided by the HTML specification
> needs to be extended or modified to ensure all accessibility use cases are
> covered. The analysis is prepared primarily by Michael Cooper, with input
> from members of the Bug Triage sub-team of the HTML Accessibility Task
> Force.
>
> This is a draft analyis put forth for discussion and should not be viewed
> as complete, final, or constituting a proposal in itself.
>
> Sections of this document:
>
>    - HTML 4 <#12c80061e2356983_html4>
>    - HTML 5 <#12c80061e2356983_html5>
>    - General issues with fallback mechanisms<#12c80061e2356983_generalissues>
>    - Use cases <#12c80061e2356983_usecases>
>
> HTML 4
>
> The table below shows known fallback mechanisms for embedded content
> elements in HTML 4. Only elements defined in HTML 4 are listed, plus the
> <embed> element which is a "de facto" HTML 4 element. The fallback
> mechanisms and issues are as generally used in the wild, not as defined by
> the specification. This is in order to identify the real support baseline
> upon which we are building.
>  Embedded content elements in HTML 4  Element / Approach @alt @title embedded
> content @longdesc @name <title> of referenced content  embed not
> standardized or widely used but recognized by some user agents   not
> standardized, but may be supported by some user agents        iframe   @title
> widely recognized as labeling the frame, but this was a narrowing of that
> general-purpose attribute     Often misunderstood and used for a text
> label, or at used in an "if all else fails" scenario, but this does not
> function well as it's a token for processing with extreme limitations Many
> advocated fetching the referenced content and using the title as a label for
> the frame, but it requires support for the element, fetching an external
> resource, and being able to find a title  img see general issues with @alt<#12c80061e2356983_general_alt> see
> general issues with @title <#12c80061e2356983_general_title>   see general
> issues with @longdesc <#12c80061e2356983_general_longdesc>      object     See
> general issues with embedded content <#12c80061e2356983_general_embedded>
>        HTML 5
>
> The table below shows fallback mechanisms documented in the HTML 5
> specification for the embedded content types supported by the specification.
> By contrast with the HTML 4 table, this HTML 5 table lists what the
> specification supports, *not* what is generally used in the wild. This is
> because the purpose of this exercise is to review the specification, not
> current usage. Note that the set of available fallback mechanisms (shown as
> column headers) is different from the set used in HTML 4.
>  Embedded content elements in HTML 5  Element / Approach @alt @title embedded
> content @longdesc @aria-labelledby @aria-describedby  audio     supported,
> after <track> elements        canvas     supported        embed
> disallowed        iframe     permitted but markup will be removed and only
> for HTML serialization; not specified if it's meant to be a fallback
> img @alt for short descriptions   not support@ted removed (under dispute) proposed
> as replacement for @alt proposed as replacement for @longdesc  math     deferred
> to features of MathML, fallbacks should be possible using @alttext
> object     supported, after <param> elements        svg     deferred to
> features of SVG; fallbacks should be possible using <desc> and <title>
>    video     supported, after <track> and potentially <source> elements
>      General issues with fallback mechanisms
>
> Some of the fallback mechanisms identified above have issues in theory or
> practice that were generic to several embedded content elements and / or
> were too long to include in the table. These issues are shown here.
> @alt
>
>    - simple and long history
>    - not internationalized and doesn't support phrase markup
>    - lack of feature to specify "decorative" images have led to various
>    heuristic uses of this attribute, but not complete universality in these
>
> @title
>
>    - variously proposed / used as a replacement and / or supplement to
>    @alt, but wildly inconsistent from an accessibility perspective
>    - primacy between @alt and @title unspecified: does @title come first,
>    last, or cover @alt? Or should be ignored from an accessibility perspective?
>
> @longdesc
>
>    - provides explicit long descriptions, contrasted with "d-link"
>    approaches that don't semantically tie description to image
>    - description referenced by a URI, but user agent behaviour not
>    specified (e.g., fetch and present? give user a link? etc.)
>    - many argue that a URI consisting only of a fragment on the same page
>    should be valid, but unclear that implementations support this use case
>    well, and expected UA behaviour not specified
>
> embedded content
>
>    - mixture of fallbacks and param elements
>    - unclear whether fallback intended for non-support of element,
>    non-support of the referenced content language (fallback only visible if the
>    technology not supported), or accessibility (fallback displayed at user
>    request)
>    - in practice, accessible fallbacks not available if browser supports
>    the technology, which is largely the case today
>
> Use cases
>
> This is an attempt to propose use cases for fallbacks, as a step towards
> understanding what use cases remain to be met. The focus is on *
> accessibility* use cases. Use cases for non-support of the embedded
> content element and non-support of the referenced content technology are
> also important, and should not be confounded with accessibility use cases.
> It may be necessary to expand on this in the future in order to cleanly
> separate them.
>
> The following list describes the use cases identified, and the table shows
> which embedded content types may have the corresponding use case. Beware of
> token overload; the terms are used here as defined below, and should not be
> interpreted to have any other meanings inherited from other contexts. Cells
> have an x where the use case is believed to apply to the embedded content
> type, otherwise they are blank.
>  Short text alternative Can substitute for the object, sometimes on its
> own and sometimes complemented by an additional long text alternative.
> Normally, short text alternatives aren't provided if direct accessibility is
> possible, but it may still be used if direct accessibility for whatever
> reason isn't enabled (e.g., canvas makes a simple image and there is no need
> to enable full shadow DOM support). Long text alternative Can substitute
> for the object, fairly completely. Normally it complements a short text
> alternative but may stand on its own, e.g., in the case of transcripts.Label Identifies
> the object and tells the user if they want to go into it more. This has both
> non-accessibility and accessibility use cases. Frequently confounded with
> short text alternatives, it's a distinct use case and optimally should have
> a different implementation. Generally, if a label is provided, a short text
> alternative would be redundant and is not separately required. Caption This
> is a visible label, such as appears below an image in many publications.
> This is not primarily an accessibility use case, but is confounded in a few
> ways: 1) implementation of the caption is often (incorrectly, in my opinion)
> taken from the implementation of short text alternatives; 2) captions and
> short text alternatives are often both provided where their content is
> identical and redundant; 3) when a caption is provided, it should also
> generally be sufficient to meet the short text alternative use case (but not
> vice versa). Summary Is more than a short text alternative, but not the
> complete replacement that a long text alternative should be. Like a label,
> it may help a user decide whether to explore more, or may be a sufficient
> overview of the object in many cases. Idiosyncratic direct accessibility The
> object content itself provides ways to make it accessible, e.g., caption
> formats in video, features of SVG, the shadow DOM of canvas, etc. Generally,
> if a format supports direct accessibility it may still benefit from a label,
> but should not require a short or long text alternative. However, some
> objects may not enable the direct accessibility and still require external
> text alternatives, such as a short text alternative for a simple image
> implemented with canvas, or a transcript (i.e., long text alternative) for
> an audio. Note that for <embed> and <object>, this depends on features of
> the loaded content language, so these elements may or may not require
> separate fallbacks within the HTML. Advisory / tooltip A kind of text
> label that is usually displayed as a tooltip. Although frequently taken from
> short text alternatives or captions, *this is not an accessibility use
> case*. It is in the table to show that it is a distinct use case and
> should not be confounded with other accessibility fallbacks. Specify none
> needed For formats that need to be able to indicate that they are
> "presentational" and no accessibility fallback is needed.   Fallback use
> cases for embedded content elements  Element / Use case short text
> alternative long text alternative label caption summary idiosyncratic
> direct accessibility advisory / tooltip specify none needed  audio   x x
>   x x x  canvas x x x x x x x x  embed x x x x x x x x  iframe     x x x
> x x  img x x   x x   x x  math x         x x x  object x x x x x x x x
> svg x   x x x x x x  video   x x     x x x
>
>
Received on Thursday, 25 November 2010 03:41:53 GMT

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