[whatwg] <source> media attribute behavior, static or dynamic ?

Paul Adenot padenot+whatwg at mozilla.com
Thu May 10 14:03:15 PDT 2012

Currently implementing the media attribute of the source element in 
Gecko, we are unsure about what the specification requires us to do there.

Considering this example :

   <source src="lowres.webm" media="max-width:500px">
   <source src="highres.webm">

what is the expected behavior of a page containing such code being 
loaded, in a page that is 1000px width, and then being resized to be 
under 500px?

Should the media currently playing change? Is the media attributes and 
the multiple <source> attribute only taken into account when the page is 

The spec [1] says :

 > Dynamically modifying a source element and its attribute when the
 > element is already inserted in a video or audio element will have no
 > effect. To change what is playing, just use the src attribute on the
 > media element directly, possibly making use of the canPlayType()
 > method to pick from amongst available resources. Generally,
 > manipulating source elements manually after the document has been
 > parsed is an unncessarily complicated approach.

which seems to indicate that the <source> element are only used at load 
time. Since |canPlayType()| is mentioned, |matchMedia()| is possibly the 
right approach here, but it is unclear to us what the spec exactly 
requires. We think the spec needs clarification here.


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