W3C home > Mailing lists > Public > public-respimg@w3.org > September 2013

Re: Simplified source selection algorithm

From: Kornel Lesiński <kornel@geekhood.net>
Date: Tue, 17 Sep 2013 19:46:43 +0100
To: "Marcos Caceres" <w3c@marcosc.com>, "Cory Brown" <oh.wise.man@gmail.com>
Cc: public-respimg@w3.org
Message-ID: <op.w3kjr5ozte2ec8@aimac.local>
On Tue, 17 Sep 2013 18:06:26 +0100, Cory Brown <oh.wise.man@gmail.com>  

>> - The algorithm picks the first <source> that matches. Matching is done 
>> based on media and type attributes.
> Agreed!
> This would match the way video source is selected, but counter to the  
> way CSS @media rules cascade. I had to address this in  
> [x-picture](https://github.com/ResponsiveImagesCG/x-picture/blob/master/js/x-picture.js#L82)  
> and leaned on last (media) match wins. Selecting the first match makes  
> the selection algorithm simpler, but would mean authors address their  
> media attributes on source elements in the reverse order they would in  
> CSS. By no means a deal breaker, but since source elements with media  
> attributes merges two paradigms, I wanted to make sure it was at least  
> considered. I suppose no matter which way it went, first or last, it  
> would be opposite one of the paradigms picture is borrowing from.

That's a very good point. Since <video> MQs are rare (might even be  
dropped from the spec) and authors are more likely to work with images+CSS  
MQ than just images+video MQ, it may be worth adjusting the order.

I don't know why, but to me it somehow makes sense that in cascade last  
override wins, but in sources first match wins (maybe I've been thinking  
too much about the implementation :)

Could we do usability study or a survey to know how typical page authors  
envision it? e.g. ask authors what they'd expect from this markup:

	<source src="blue.png">
	<source src="orange.png">

regards, Kornel
Received on Tuesday, 17 September 2013 18:47:16 UTC

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