W3C home > Mailing lists > Public > www-style@w3.org > November 2014

Re: [mediaqueries] Media Source Extensions and device capabilities

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Tue, 18 Nov 2014 09:45:53 -0800
Message-ID: <CAAWBYDDhLskHKdkNAi=JaXythA0mvucd2griS2xN-gDrZ+EcZw@mail.gmail.com>
To: "L. David Baron" <dbaron@dbaron.org>
Cc: Florian Rivoal <florian@rivoal.net>, Mark Watson <watsonm@netflix.com>, "www-style@w3.org" <www-style@w3.org>
On Tue, Nov 18, 2014 at 9:06 AM, L. David Baron <dbaron@dbaron.org> wrote:
> On Tuesday 2014-11-18 14:59 +0100, Florian Rivoal wrote:
>> > On 14 Nov 2014, at 18:13, L. David Baron <dbaron@dbaron.org> wrote:
>> > Then again, I probably wouldn't be against just having a separate
>> > unit for device pixels (or, for that matter, for true lengths),
>> > although I'm not sure I like 'dpx' as the name (looks too much like
>> > 'dpi’).
>> We already have dppx (Dots Per PiXels) for the resolution MQ, to query the number of physical pixels in a css pixels, so I don’t think dpx is a name I’d recommend.
>> As for physical units for lengths, coming from mostly anyone else, I’d probably reply with the usual story about why this is misguided, but as you know all these already, I am curious as to why you think this is a potentially good idea.
> I think it's a potentially good idea here because what we're talking
> about is media resource selection.
> Consider the problem as follows:  the server can offer a video at
> four different resolutions, ranging from, say, 320x180 up to
> 1280x720.  (I'm making up these details, but this is the general
> idea.)  They want to offer the highest quality one that's going to
> be useful, but don't want to waste bandwidth sending the high
> quality one to a small display.
> So the right choice of video is a function of how many *device
> pixels* wide and tall the space for the video is.  It's not a
> function of CSS pixels; treating it as such would send low quality
> video to displays with more than one device pixel per CSS pixel.

This problem seems identical to that of <picture>, and particularly
the sizes/w syntax, where you don't know precisely how wide, in CSS
px, the image will end up being displayed, so you can't tell what the
density of any given source file will be (since the density is image
pixels / CSS px).

The solution is not to use MQs, because that requires a bunch of cases
(which multiply as you have different sizes), and requires guessing at
what point switching over to a different video source might be better.
(We have some examples of the kind of markup required to serve a
couple of different image sources to devices adequately.  It's... not
pretty.)  The right solution is to tell the browser how many pixels
wide the source is, and how wide it'll end up, so the browser can
figure out the effective density for you, and choose the correct

This might be something we need to solve in HTML, by adopting
something analogous to srcset.  Or, if we're okay with letting CSS
override <video> source selection, we can solve it with something
parallel to image-set(), specialized for videos.

Audio output capability is still something that can be addressed via
MQs.  (Though it sounds like offering the number of audio channels
directly via some JS API would also be useful to Mark Watson's

Received on Tuesday, 18 November 2014 17:46:57 UTC

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