[whatwg] Google Feedback on the HTML5 media a11y specifications

Philip Jägenstedt philipj at opera.com
Sat Jan 22 06:23:19 PST 2011

On Fri, 14 Jan 2011 10:01:38 +0100, Silvia Pfeiffer  
<silviapfeiffer1 at gmail.com> wrote:

> There are two sections - the first one concerns the WebVTT format and
> the second one the <track> specification.

Thanks for compiling all of this feedback, Silvia! As usual, my inline  
replies are sometimes terse, not to be mistaken for impatience or  
disrespect :)

> A. Feedback on the WebVTT format

> 1. Introduce file-wide metadata
> WebVTT requires a structure to add header-style metadata. We are here
> talking about lists of name-value pairs as typically in use for header
> information. The metadata can be optional, but we need a defined means
> of adding them.
> Required attributes in WebVTT files should be the main language in use
> and the kind of data found in the WebVTT file - information that is
> currently provided in the <track> element by the @srclang and @kind
> attributes. These are necessary to allow the files to be interpreted
> correctly by non-browser applications, for transcoding or to determine
> if a file was created as a caption file or something else, in
> particular the @kind=metadata. @srclang also sets the base
> directionality for BiDi calculations.

Are there non-browsers that use the language for font-selection or bidi?  
Is auto-detection not likely to give a better user experience? Are there  
any other use cases for knowing the language of the captions *after*  
they've been opened?

Why do non-browser players need to know the kind? All kinds are processed  
in the same way except metadata, and there's no reason to use metadata  
tracks with external players.

> Further metadata fields that are typically used by authors to keep
> specific authoring information or usage hints are necessary, too. As
> examples of current use see the format of MPlayer mpsub’s header
> metadata [2], EBU STL’s General Subtitle Information block [3], and
> even CEA-608’s Extended Data Service with its StartDate, Station,
> Program, Category and TVRating information [4]. Rather than specifying
> a specific subset of potential fields we recommend to just have the
> means to provide name-value pairs and leave it to the negotiation
> between the author and the publisher which fields they expect of each
> other.

This approach has worked very well with Vorbis Comments, probably mostly  
because all interesting fields have been pre-defined in  

For a web format though, wouldn't some kind of wiki registry be good to  
avoid total mayhem, especially if there are some predefined fields? (Not  
having file-wide metadata would also avoid such mayhem.)

> 2. Introduce file-wide cue settings
> At the moment if authors want to change the default display of cues,
> they can only set them per cue (with the D:, S:, L:, A: and T:. cue
> settings) or have to use an external CSS file through a HTML page with
> the ::cue pseudo-element. In particular when considering that all
> Asian language files would require a “D:vertical” marker, it becomes
> obvious that this replication of information in every cue is
> inefficient and a waste of bandwidth, storage, and application speed.
> A cue setting default section should be introduced into a file
> header/setup area of WebVTT which will avoid such replication.
> An example document with cue setting defaults in the header could look
> as follows:
> ==
> Language=zh
> Kind=Caption
> CueSettings= A:end D:vertical
> 00:00:15.000 --> 00:00:17.950
> 在左边我们可以看到...
> 00:00:18.160 --> 00:00:20.080
> 在右边我们可以看到...
> 00:00:20.110 --> 00:00:21.960
> ...捕蝇草械.
> ==
> Note that you might consider that the solution to this problem is to
> use external CSS to specify a change to all cues. However, this is not
> acceptable for non-browser applications and therefore not an
> acceptable solution to this problem.

Indeed, repeating settings on each cue would be annoying. However,  
file-wide settings seems like it would easily be too broad, and you'd have  
to explicitly reverse the effect on the cues where you don't want it to  
apply. Maybe classes of cue settings or some kind of macros would work  

Nitpick: Modern Chinese, including captions, is written left-to-right,  
top-to-bottom, just like English.

> 3. Cue settings requirements

> * naming: The usage of single letter abbreviations for cue settings
> has created quite a discussion here at Google. We all agree that
> file-wide cue settings are required and that this will reduce the need
> for cue-specific cue settings. We can thus afford a bit more
> readability in the cue settings. We therefore believe that it would be
> better if the cue settings were short names rather than single letter
> codes. This would be more like CSS, too, and easier to learn and get
> right. In the interface description, the 5 dimensions have proper
> names which could be re-used (“direction”, “linePosition”,
> “textPosition”, “size” and “align"). We therefore recommend replacing
> the single-letter cue commands with these longer names.
> An example document with more verbose cue settings could look as follows:
> ==
> Language=zh
> Kind=Caption
> CueSettings= align:end direction:vertical
> 00:00:15.000 --> 00:00:17.950 linePosition:80%
> 在左边我们可以看到...
> 00:00:18.160 --> 00:00:20.080
> 在右边我们可以看到...
> 00:00:20.110 --> 00:00:21.960 size:70%
> ...捕蝇草械.
> ==

I agree, every time I see the single-letter settings I have to go look at  
the spec to figure out what they mean. I'd be happy to have more explicit  
names. I'd be even happier if they match CSS terminology where possible.

> 4. Cue formatting requirements
> In analysing the available cue formatting functionality, we have found
> that some features are missing. Most of these features can be added
> through using CSS on cues that have received a <b>, <i>, <c> or <v>
> marker. The following features are core to traditional TV and exist in
> EBU STL and CEA-608/708 captions. Support of these will be a core
> requirement for browsers as well as non-browser applications and it
> makes sense to add these to WebVTT rather than relying on external CSS
> which cannot be used for non-browser captions:

The unstated requirement here seems to be that WebVTT needs to work as an  
interchange format for various TV captioning formats even in user agents  
without any support for CSS (or JavaScript). I'm trying to not make a  
straw man argument, but if want an interchange format, we should pick  
TTML, which is explicitly designed to be just that and doesn't depend on  

Is it not enough that a lossy conversion can be made from various formats  
into WebVTT+CSS(+JavaScript)? If not, the "Web" in "WebVTT" is highly  

> * textcolor: In particular on European TV it is common to distinguish
> between speakers by giving their speech different colors. The
> following colors are supported by EBU STL, CEA-608 and CEA-708 and
> should be supported in WebVTT without the use of external CSS: black,
> red, green, yellow, blue, magenta, cyan, and white. As default we
> recommend white on a grey transparent background.

What's wrong with <v Speaker>? If a completely automatic conversion is  
needed, why not <c.yellow>...</c>? Both methods have the distinct  
advantage of making it easy to change or disable the colors with only CSS  

> * underline: EBU STL, CEA-608 and CEA-708 support underlining of
> characters. The underline character is also particularly important for
> some Asian languages. Please make it possible to provide text
> underlines without the use of CSS in WebVTT.

Which Asian languages? If it's just the Chinese  
<http://en.wikipedia.org/wiki/Proper_name_mark>, then I don't think that  
needs <u> or similar. In my experience, use of the Chinese proper name  
mark is in fact extremely rare in Chinese captions, at least in movies and  
TV series from the mainland and Taiwan. It would be best to use e.g.  
我來自<c.pnm>中國</c> to make it easy to change the style between  
single/double/wavy/no underline.

> * blink: As much as we would like to discourage blinking subtitles,
> they are actually a core requirement for EBU STL and CEA-608/708
> captions and in use in particular for emergency messages and similar
> highly important information. Blinking can be considered optional for
> implementation, but we should allow for it in the standard.

00:00.000 --> 00:00.500

00:01.000 --> 00:02.500

00:02.000 --> 00:02.500

Is that enough? In the context of the web there are much better ways to  
convey very import information than through blinking captions. Event  
alert() would be better.

> * font face: CEA-708 provides a choice of eight font tags: undefined,
> monospaced serif, proportional serif, monospaced sans serif,
> proportional sans serif, casual, cursive, small capital. These fonts
> should be available for WebVTT as well. Is this the case?

Does the choice of font ever carry any semantic meaning? Isn't it a good  
thing that captions can't specify their own fonts, so that it's easy to  
pick a style that's suitable for the embedding site?

> [On a side note, we wonder if it would make sense to introduce an
> @kind=”annotation” type of TimedText track, which can then allow full
> innerHTML markup be rendered on top of the video viewport. This would
> probably need to be matched with full CSS support, too. It would allow
> people to introduce unconventional caption display such as captions in
> speech bubbles that can track the characters as they move or know
> about what important objects are on the screen, so never overlap them.
> Note that script in innerHTML needs to be dealt with carefully to
> avoid XSS attacks. @kind=”annotation” is not required for ordinary
> captions, so we have not investigated this need in full detail.]

Won't Implement ;) For reasons already discussed at length, I think HTML  
in captions is a bad idea. Having *both* WebVTT cue text parsing and  
innerHTML parsing would be even more complicated, though.

> 5. Markup changes
> We have a couple of recommendations for changes mostly for aesthetic
> and efficiency reasons. We would like to point out that Google is very
> concerned with the dense specification of data and every surplus
> character, in particular if it is repeated a lot and doesn’t fulfill a
> need, should be removed to reduce the load created on worldwide
> networking and storage infrastructures and help render Web pages
> faster.

Nipick: Is network load really an issue here? Compared to the video files  
they accompany, caption files are tiny, even more so with gzip/deflate.

> * Time markers: WebVTT time stamps follow no existing standard for
> time markers. Has the use of NPT as introduced by RTSP[5] for time
> markers been considered (in particular npt-hhmmss)?
> [5] http://www.ietf.org/rfc/rfc2326.txt

Unfortunately, the hour component is not optional in NPT. Also, the  
decimal part of seconds is of arbitrary precision, which doesn't seem  

> * Suggest dropping “-->”: In the context of HTML, “-->” is an end
> comment marker. It may confuse Web developers and parsers if such a
> sign is used as a separator. For example, some translation tools
> expect HTML or XML-based interchange formats and interpret the “>” as
> part of a tag. Also, common caption convention often uses “>” to
> represent speaker identification. Thus it is more difficult to write a
> filter which correctly escapes “-->” but retains “>” for speaker ID.

Trying to use an HTML or XML parser to make any sense of WebVTT is going  
to fail horrendously in any case, so if anything I think it's good that  
they fail early. Also, a translation tool that has no concept of WebVTT is  
going to make a mess of various magic strings used in the file format too.

> Since the “-->” characters serve no obvious purpose, it should be
> possible to safely replace them by a blank that separates start and
> end time, thus making the format denser and removing annoying parsing
> issues. (Or alternatively use a the npt-range spec of RTSP for time
> ranges, which uses “-” as a separator.).

No strong opinion, but I think a non-blank separator is more aesthetically  

> * Duration specification: WebVTT time stamps are always absolute time
> stamps calculated in relation to the base time of synchronisation with
> the media resource. While this is simple to deal with for machines, it
> is much easier for hand-created captions to deal with relative time
> stamps for cue end times and for the timestamp markers within cues.
> Cue start times should continue to stay absolute time stamps.
> Timestamp markers within cues should be relative to the cue start
> time. Cue end times should be possible to be specified either as
> absolute or relative timestamps. The relative time stamps could be
> specified through a prefix of “+” in front of a “ss.mmm” second and
> millisecond specification. These are not only simpler to read and
> author, but are also more compact and therefore create smaller files.
> An example document with relative timestamps is:
> ==
> Language=en
> Kind=Subtitle
> 00:00:15.000   +2.950
> At the left we can see...
> 00:00:18.160    +1.920
> At the right we can see the...
> 00:00:20.110   +1.850
> ...the <+0.400>head-<+0.800>snarlers
> ==

I rather like it, although it might be confusing if "-" means "to absolute  
time" and "+" means "to relative time". That the intra-cue timings are  
relative but the timing lines are absolute has bugged me a bit, so if the  
distinction was more obvious just from the syntax, that'd be great!

> 6. Format identifier
> We are happy to see the introduction of  the magic file identifier for
> WebVTT which will make it easier to identify the file format. We do
> not believe the “FILE” part of the string is necessary.

I agree, mostly because it's ugly. While we're bikeshedding, "WebSRT" is  
prettier than "WEBSRT".

> However, we
> recommend to also introduce a format version number that the file
> adheres to, e.g. “WEBVTT 0.7”. This helps to make non-browser systems
> that parse such files become aware of format changes. It can also help
> identify proprietary standard metadata sets as used by a specific
> company, such as “WEBVTT 0.7 ABC-meta1” which could signify that the
> file adheres to WEBVTT 0.7 format specification with the ABC-meta1
> metadata schema. Parsers are then made aware of what fields to expect
> and can alert human operators of unexpected fields or markup.
> Browsers can safely ignore such a marker and instead do a best effort
> on parsing based on what they understand.

I strongly disagree, WebVTT shouldn't have a version indicator for the  
same reasons that HTML, CSS and JavaScript don't. Making proprietary  
extensions easier to maintain should be an anti-goal.


> we recommend the introduction of comments.

I agree and think it needs to happen before WebVTT starts to get  
implemented and used on the web. In other words: now.

> 8. Line wrapping
> CEA-708 captions support automatic line wrapping in a more
> sophisticated way than WebVTT -- see
> http://en.wikipedia.org/wiki/CEA-708#Word_wrap.
> In our experience with YouTube we have found that in certain
> situations this type of automatic line wrapping is very useful.
> Captions that were authored for display in a full-screen video may
> contain too many words to be displayed fully within the actual video
> presentation (note that mobile / desktop / internet TV devices may
> each have a different amount of space available, and embedded videos
> may be of arbitrary sizes). Furthermore, user-selected fonts or font
> sizes may be larger than expected, especially for viewers who need
> larger print.
> WebVTT as currently specified wraps text at the edge of their
> containing blocks, regardless of the value of the 'white-space'
> property, even if doing so requires splitting a word where there is no
> line breaking opportunity. This will tend to create poor quality
> captions.  For languages where it makes sense, line wrapping should
> only be possible at carriage return, space, or hyphen characters, but
> not on   characters.  (Note that CEA-708 also contains
> non-breaking space and non-breaking transparent space characters to
> help control wrapping.)However, this algorithm will not necessarily
> work for all languages.
> We therefore suggest that a better solution for line wrapping would be
> to use the existing line wrapping algorithms of browsers, which are
> presumably already language-sensitive.
> [Note: the YouTube line wrapping algorithm goes even further by
> splitting single caption cues into multiple cues if there is too much
> text to reasonably fit within the area. YouTube then adjusts the times
> of these caption cues so they appear sequentially.  Perhaps this could
> be mentioned as another option for server-side tools.]

Yeah, with SRT people are manually line-wrapping when authoring the  
captions and often enough the end result is that you get something  

- Who could have guessed that not all fonts are the same
- That's news to me, so I get four lines of text where I
wanted two!

I'm inclined to say that we should normalize all whitespace during parsing  
and not have explicit line breaks at all. If people really want two lines,  
they should use two cues. In practice, I don't know how well that would  
fare, though. What other solutions are there?

> B. Feedback on the <track> element
> 1. Pop-on/paint-on/roll-up support
> Three different types of captions are common on TV: pop-on, roll-up
> and paint-on. Captions according to CEA-608/708 need to support
> captions of all three of these types. We believe they are already
> supported in WebVTT, but see a need to re-confirm.

The underlying use case here is live captioning, right? Just copying the  
styling used on broadcast TV seems like it wouldn't be enough, you also  
need the ability to erase typos, right? Are there any existing captioning  
formats that handle live captioning well from which one could draw  

> 2. Duplicate track
> The HTML spec specifies that it is not allowed to have two tracks that
> provide the same kind of data for the same language (potentially
> empty) and for the same label (potentially empty). However, we need
> clarification on what happens if there is a duplicate track, ie: does
> the most recent one win or the first one or will both be made
> available in the UI and JavaScript? The spec only states that the
> combination of {kind, type, label} must be unique. It doesn't say what
> happens if they are not.

In <http://whatwg.org/html#sourcing-out-of-band-text-tracks> all track are  
added to the list of text tracks, even duplicates.

In other words, it's just a requirement for validators, not user agents.

> Further, the spec says nothing about duplicate labels altogether -
> what is a browser supposed to do when two tracks have been marked with
> the same label?

We'd show the same text in the context menu and let the user be confused,  
I guess. It's very easy for authors who care about not confusing their  
users to fix, so I don't think browsers need to be clever here.

> 4. Addressing individual cues through CSS
> As far as we understand, you can currently address all cues through
> ::cue and you can address a cue part through ::cue-part(<voice> ||
> <part> || <position> || <future-compatibility>). However, if we
> understand correctly, it doesn’t seem to be possible to address an
> individual cue through CSS, even though cues have individual
> identifiers. This is either an oversight or a misunderstanding on our
> parts. Can you please clarify how it is possible to address an
> individual cue through CSS?

Since I've been arguing against the id's in WebVTT, I'm curious about the  
use case here. Isn't using a unique class good enough?

> 5. Ability to move captions out of the way
> Our experience with automated caption creation and positioning on
> YouTube indicates that it is almost impossible to always place the
> captions out of the way of where a user may be interested to look at.
> We therefore allow users to dynamically move the caption rendering
> area to a different viewport position to reveal what is underneath. We
> recommend such drag-and-drop functionality also be made available for
> TimedTrack captions on the Web, especially when no specific
> positioning information is provided.

This would indeed be rather nice, but wouldn't it interfere with text  
selection? Detaching the captions into a floating, draggable window via  
the context menu would be a theoretically possible solution, but that's  
getting rather far ahead of ourselves before we have basic captioning  

Philip Jägenstedt
Core Developer
Opera Software

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