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Re: [selectors] Time-dimensional pseudo-classes and multiple timelines?

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Mon, 8 Dec 2014 13:37:49 -0800
Message-ID: <CAAWBYDBOqAQKKnBPLcpn364xS2sztTF7OZNUyvC542LTf9Ht3A@mail.gmail.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: www-style list <www-style@w3.org>
On Mon, Dec 8, 2014 at 1:18 PM, Josh Triplett <josh@joshtriplett.org> wrote:
> On Mon, Dec 08, 2014 at 11:34:07AM -0800, Tab Atkins Jr. wrote:
>> On Mon, Dec 1, 2014 at 11:52 AM, Josh Triplett <josh@joshtriplett.org> wrote:
>> > The examples given for the time-dimensional pseudo-classes (:current,
>> > :past, :future) mention speech rendering of a document, which could
>> > potentially apply to either built-in functionality of a page or to
>> > third-party assistance software.  Other CSS specifications/drafts (such
>> > as css2/2.1's aural stylesheets, or css3-speech) define CSS properties
>> > that apply specifically to speech, and mention both accessibility
>> > software and several other applications.
>> >
>> > In the case of those speech-related properties, typically one property
>> > value would make sense for a given element; for instance, the CSS
>> > associated with an audiobook could associate a given voice with a
>> > particular character's lines.
>> >
>> > However, in the case of the time-dimensional pseudo-classes, it could
>> > make sense to have multiple orthogonal users of these classes.  For
>> > instance, one set of rules using these pseudo-classes might make sense
>> > for use with assistive software, while another would make sense when
>> > using those classes for a slide presentation.
>> >
>> > Given that flexibility, would it make sense to add a cross-reference to
>> > CSS media types ("@media speech", "@media projection") as one possible
>> > mechanism to apply different time-dimensional rules for different use
>> > cases?  Something like this (with "CSS media types" as a link to
>> > http://www.w3.org/TR/CSS21/media.html):
>> >
>> > """
>> > Multiple timelines may apply to a given document; for instance, a
>> > document presented via speech rendering may define a timeline based on
>> > the current position in that rendering, while a document presented as a
>> > series of projected slides may define a timeline based on the currently
>> > projected slide and the presenter's current position in that slide.
>> > Authors may wish to use CSS media types (e.g. "@media speech", "@media
>> > projection") to apply different rules using time-dimensional
>> > pseudo-classes to different types of timelines.
>> > """
>> >
>> > Also, an additional example using several of these pseudo-classes to
>> > style slides using incrementally revealed points, as well as
>> > illustrating the concept of :current matching ancestors of the current
>> > element:
>> >
>> > """
>> > @media projection {
>> >   .slide:matches(:past, :future) { display: none; }
>> >   .slide:current { display: block; background-color: white; }
>> >   .slide:current li:current { color: red; }
>> >   .slide:current :future { opacity: 0.3; }
>> > }
>> >
>> > This hides all slides except the current slide, highlights the current
>> > point in red, and fades out future material.
>> > """
>> >
>> > (I added "background-color: white;" to .slide to avoid having the
>> > example contradict http://www.w3.org/QA/Tips/color .)
>>
>> While all of this is a good idea in theory, currently the only user of
>> the time-dimensional pseudo-classes is in styling WebVTT subtitles via
>> the ::cue pseudo-element.  While it's usually fine to add speculative
>> examples for features that haven't yet been implemented, it's unclear
>> to me if examples like what you provide will *ever* exist, and I think
>> putting in examples that imply they do or will might confuse people
>> more than it would illuminate things.
>
> Fair enough; no sense putting in the "@media projection" example then.
> (I'd imagined it as something supported by HTML5-based presentation
> software, perhaps via a polyfill.)
>
> However, despite the current use of this solely for WebVTT, I think it'd
> be unfortunate if this had a generic (non-WebVTT-specific) name but
> there could *never* be any other use of these selectors due to conflicts
> between multiple uses.  So even without a projection-specific example,
> could you add something providing guidance for the multiple-timeline
> case?

Again, until browsers show some kind of interest in developing this,
I'm uncomfortable in guessing on how it'll work.  We can work out the
conflicts when people are interested enough in the base feature for
the conflicts to be relevant. ^_^

>> > Finally, I think it would make sense to elaborate a bit further on the
>> > behavior of :current, :past, and :future with respect to ancestor
>> > elements: :current applies to all ancestors of a :current element (all
>> > the way to :root), but :future and :past do not, precisely because
>> > :current does.  Any given element must match at most one of :current,
>> > :past, or :future.
>>
>> Yeah, that's definitely the intent.  Added.
>
> Thanks!
>
> Where does the current in-progress spec live, with the text you added?
> Is there a version control repository somewhere, or a snapshot newer
> than the 26 November 2014 version?  (I didn't see any references to
> either on http://dev.w3.org/csswg/selectors4/ .)

That latter one is the live spec, I've just got some repository
conflicts right now, so I haven't pushed the commit yet.  Don't worry.
^_^

~TJ
Received on Monday, 8 December 2014 21:38:36 UTC

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