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[webvtt] APA Comments on TimedText/WebVTT

From: Janina Sajka <janina@rednote.net>
Date: Tue, 30 Jan 2018 10:55:56 -0500
To: public-tt@w3.org
Cc: Accessible Platform Architectures Administration <public-apa-admin@w3.org>, W3C WAI Accessible Platform Architectures <public-apa@w3.org>, David Singer <singer@apple.com>
Message-ID: <20180130155556.GG15026@rednote.net>

As requested here:

And further logged at:

The Accessible Platform Architectures (APA) Working Group has reviewed
TimedText/WebVTT and offers the following comments.

According to APA process, the formal APA decision on these comments is
logged at:

Janina Sajka, APA Chair

<Begin Comments>

*Item #1 Users of Magnification:*

The bug-tracker indicates the following response(s):

  * Snap to flag (assumption: snap-to-lines flag)

     -> Concern: the concern is likely that if text is explicitly
positioned on certain lines, there is potential that enlarging the text may
lead to text growing outside the rendering area (e.g. text positioned in
the top-most line), or successive lines may grow into each other.

     -> Reply: First we have to understand that when content is enlarged,
the video is typically enlarged together with the text. Therefore, the
occurrance of this problem is rare. Secondly, there is a conflict between
authoring requirements and rendering limitations. The browser rendering
algorithm will [adjust?] as well as it can, but once text breaks out of the
video rendering boundaries, there is not much it can do.

     -> APA Response: APA recognizes the technical constraints noted here
with regard to rendering limitations. Authoring Guidance recommendations
should nonetheless indicate the potential of this problem, and urge content
authors to strive to have captions (etc.) be no greater than 50% of the
default width of the viewport (which would allow for a text increase of
roughly 200% without clipping). APA notes that for low-vision users, even
at full-screen, those users may still need to enlarge the caption text to
meet their reading needs.

  * Sizing of the captions rendering area

     -> Concern: the concern is likely that the display area of captions is
limited to the background area of the video element it is rendered onto and
that with magnification the captions may go outside this rendering area.

     -> Reply: The area outside the video element is no usable to render
captions onto (think about full-screen mode, or if the video is on a Web
page there is other content around the video element). Therefore, after
having done all it can to try and retain visibility of all caption text,
the browser will hit the limit of what it can do.

     -> APA Response: APA again recognizes the technical constraints noted
here with regard to rendering limitations. We once again recommend good
authoring guidance to ensure that content authors are aware of the
potential issue raised, so that authoring decisions regarding line-lengths
and amount of caption text rendered on screen at any single instance can be
made with this knowledge.

  * Visibility of captions when text is zoomed

     -> Concern: the concern is likely about what happens when the text is
zoomed, but the video isn't.

     -> Reply: If there are tools that do this, then you will hit the
issues of overlapping text and disappearing text when hitting the
boundaries of the rendering area faster than normal. However, it is
unlikely that a tool would exist that zooms just the text and not the video
element on screen. Normally, all content on a Web page is zoomed when
magnification or zooming tools are in use.

     -> APA Response: It was APA's understanding that one of the benefits
of WebVTT was that it could be further styled by the content author using
CSS. User stylesheets today provide the ability for users to modify and
enlarge onscreen text, and tools and browser extensions exist today to
accomplish this task.

The presumption that video content would be zoomed to the same level of
caption text is, from APA's perspective, unfounded and incorrect, and the
emergent WCAG 2.1 specifically will have a new Success Criteria (Success
Criterion 1.4.12 Text spacing -
which currently notes that caption files (when supplied as stand-alone
time-stamped documents) are covered by this SC. Please also see:

> Normally, all content on a Web page is zoomed when magnification or
zooming tools are in use.

Respectfully, this is factually incorrect. Browser-based zoom traditionally
operates like this, however other Assistive Technology tools may only zoom
a part of the whole web page, or may only apply zoom to text (and/but not
imagery). Some user-agents and platforms also allow for end-user font
magnification (for example, on the Android platform, individual users can
choose from different default font sizes, that are applied to all on-screen

APA again recognizes the technical limitations noted here with regard to
rendering limitations, and strongly recommends that appropriate authoring
guidance to address all 3 related issues noted here be included (directly)
in the Recommendation.


*Item #2: The spec should include feature explanations in plain language*

     -> Reply: no change, we rely on external documents to provide an
authoring guide.

     -> APA Response: There are actually 2 responses here.

The first is related to an on-going request from APA that W3C Technical
Recommendations also include, when and where necessary, prose summaries in
"plain english" that explain features and functions of the various parts
of any given spec. The intent here is to explain what is happening with the
technology in lay terms, rather than explain how to create content using
the technology. (i.e.: don't just show an API [sic], explain it.) This
remains an important request from APA, but is not a blocking issue.

The second response is related to Authoring Guidance documents (referenced
in your reply). Following up on the Bug Tracker, it shows a link to an
authoring guidance document (
https://docs.webplatform.org/wiki/concepts/VTT_Captioning), yet that URL
returns a 404 today.

   - Has this document been relocated elsewhere? If yes, can we please have
   the reference URL. If no, does the WG plan on updating/recreating this
   document? (This also ties-back to Item #1)

   - The current WebVTT Working Draft (https://www.w3.org/TR/webvtt1)
   currently has 'editorial guidance' as part of the normative specification
   addressing privacy and security, and APA's request is that editorial
   guidance for accessibility considerations also be provided in the same
   fashion (i.e. directly in the Rec, as opposed to linking out.)

APA would be pleased to assist in the review of any authoring guidance that
emerges from the WebVTT WG.


*Item #3: Captions on the audio element*

     -> Reply: fixed, explanations added.

     -> APA Response: Thank you.

APA trusts this meets your request, and we are happy to further elaborate
on any of these issues as required.

<End Comments>


Janina Sajka

Linux Foundation Fellow
Executive Chair, Accessibility Workgroup:	http://a11y.org

The World Wide Web Consortium (W3C), Web Accessibility Initiative (WAI)
Chair, Accessible Platform Architectures	http://www.w3.org/wai/apa
Received on Tuesday, 30 January 2018 15:56:53 UTC

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