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Re: Media Queries and optimizing what data gets transferred

From: Yoav Weiss <yoav@yoav.ws>
Date: Wed, 30 Jan 2013 16:04:29 +0100
Message-ID: <CACj=BEhcbgbBXSaQ9FunrUOXvzWaYMUmV8FPa_6EUopSa2o8MA@mail.gmail.com>
To: Ilya Grigorik <ilya@igvita.com>
Cc: Henri Sivonen <hsivonen@iki.fi>, www-style list <www-style@w3.org>
Just a short comment regarding the markup based solutions that are
currently on the table.
The picturefill example you referred to does not represent the latest
<picture>/srcset proposals.
A similar markup, assuming the different display size images are different
(i.e. the "art-direction" use-case) would look like:

      <source src="small.jpg" media="(max-width: 399px)" />
<source srcset="medium.jpg 1x, medium_x2.jpg 2x" media="(min-width:
400px)" />
      <source srcset="large.jpg 1x, large_x2.jpg 2x"
media="(min-width: 800px)" />
      <source srcset="extralarge.jpg 1x, extralarge_x2.jpg 2x"
media="(min-width: 1000px)" />      A giant stone face at The Bayon
temple in Angkor Thom, Cambodia

Not extremely terse, but simpler than the syntax linked to earlier.
Also, I feel I should note that markup can also be easily automated
using various templating solutions,
so a markup based solution does not imply manually typing each tag.

With that said, I don't think a "Media Queries vs. Client hints" debate is
IMO both client-side & server-side solutions have a place, each with its
slightly different use-cases and slightly different trade-offs.


On Sun, Jan 27, 2013 at 12:38 AM, Ilya Grigorik <ilya@igvita.com> wrote:

> Henri, thanks for starting this thread.
> Since I'm the one proposing Client-Hints, let me try to address some of
> the misconceptions:
> (1) It is *not* a question either/or. To the extent possible, both a
> client-side and a server-side solution should be available for content
> adaptation. Posing Client-Hints as "breaking" or impeding client-side
> adaption misses the point entirely.
> (2) Some things are far better handled by the server, not by the client.
> Things like resizing images to optimal width and height, in the world of
> ever exploding form factors is easily automated at the server level, and
> leads to massive bloat on the client. I don't mean to pick on picturefill,
> but just for the sake of an example:
> https://github.com/scottjehl/picturefill#hd-media-queries
> The above only covers 3 breakpoints and HiDPI. Do you really expect
> everyone will start filling their pages with that or similar markup for
> *every*  image asset on the page? What if we also add a connection type
> constraint into the equation? I think you see where I'm heading with this.
> A case in point is mobile proxies deployed by some of the carriers around
> the world: they resize the images on the fly, and in the process achieve
> *huge* bandwidth savings. Also, note that resizing images does not have to
> come at cost to quality -- especially if you're the one controlling how
> they get scaled and at which quality. </tangent>
> Having said that, images is just one example. Client-Hints is a generic,
> cache-friendly transport for client-server negotiation. Due to lack of such
>  mechanism, most people rely on cookies today, which is a disaster: not
> cache friendly, doesn't work cross-origin, forces requests to origin
> servers. Alternatively, if not cookies, the you have to buy a commercial
> device database (device atlas, wurfl, etc).
> In fact, I think your conclusions are almost entirely incorrect:
>> Reasons why I think introducing an HTTP-based solution would be worse
>> than adjusting CSS include:
>>  * HTTP caches don't know what the negotiation logic is, so they need
>> to check with the origin server for each Client-Hints header value
>> that they don't already have a cache key.
> Not true. The whole point of Client-Hints is to enable caches to perform
> "Vary: Client-Hints". What you've described is how the process works
> today... the requests are forced down to origin because we don't have a
> clean cache key to vary on.
>  * If the origin server doesn't get ETags right, intermediate caches
>> end up having a distinct copy of the data for each distinct
>> Client-Hints header value even if there is a smaller number of
>> different data alternatives on the origin server.
> Etags has *nothing* to do with this, and ETags is also not a mechanism to
> vary different responses to begin with.
>>  * Pushing the resource selection logic in the browser and presenting
>> the browser with different URLs to choose from allows HTTP
>> intermediaries to operate simply with URL cache keys. Also, no special
>> logic is needed on the origin server.
> Correct. See my earlier comment about supporting both client and
> server-driven negotiation. They are not mutually exclusive.
>>  * Sending any HTTP header incurs extra traffic for all the sites that
>> don't pay attention to Client-Hints. That would be the whole Web at
>> least at first. That is, an HTTP-based solution involves a negative
>> externality for non-participating sites.
> This is easily addressed by making it an opt-in mechanism for HTTP 1.1. An
> equivalent mechanism to "Alternate-Protocol" can be added and Client-Hints
> would only be triggered for sites that want this data. For HTTP 2.0, with
> header deltas the overhead is a single header line.
> Further, the "cost" of upstream bytes, which is in the dozens of bytes, is
> easily offset by saving hundreds of kilobytes in the downstream (in case of
> images). The order of magnitude delta difference is well worth it.
>>  * If we later find out that Client-Hints hasn't become popular enough
>> to justify the extra traffic, we will be unable to unbloat the HTTP
>> requests, because there's always *some* site that would break in the
>> face of such unbloating.
> See my comment about opt-in.
>>  * It's bad to have to add server-side logic when CSS almost has the
>> feature authors want but not exactly.
> Not true. From first hand experience with PageSpeed, we know that despite
> continuous reminders web authors are simply not optimizing their images
> correctly: wrong formats, wrong sizes, etc. Automation solves this problem.
> While not 100% related to this discussion, see my post here:
> http://www.igvita.com/2012/12/18/deploying-new-image-formats-on-the-web/
>>  * It's conceptually bad in terms of the learnability of the Web
>> Platform that a slight adjustment to the desired behavior would
>> involve changing the solution the author needs to apply to a
>> completely different technology in the stack (from MQ to HTTP
>> negotiation).
> Once again, they're not exclusive. If you don't have a server that can
> support image optimization, you should be able to hand-tune your markup.
> I'm all for that.
> ig
Received on Wednesday, 30 January 2013 15:04:57 UTC

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