[whatwg] Implementation complexity with elements vs an attribute (responsive images)

[Simon Pieters]

There seem to be two proposals for what syntax to use for the responsive  
images use case: several elements vs. an attribute.

I think an attribute is simpler to implement and thus likely to result in  
fewer bugs in browsers, which in turn benefits Web developers.

With <img src="..." srcset="...">, in the parser case, all the information  
is available when the tag has been parsed. Excellent! In the scripted  
case, we need to "await a stable state" when the src or srcset attributes  
are changed (because the same script might also change the other), and  
then process the attributes again. The spec already does this for src=""  
because of the crossorigin="" attribute, so it's basically a solved  
problem already for <img> in the spec.

With <picture><img src="..."><source ...>...</picture>, in the parser  
case, the browser can't know it has all the information is not available  
until the picture element has been popped off the stack (since there might  
be futher <source> elements), which necessarily delays fetching the image.  
Moreover, scripts might have changed things before then, because scripts  
can run between the time that the "picture" element is added to the  
document and the time it is popped off the stack, which is a potential  
source for bugs. In the scripted case, we need to listen for changes to  
img elements' parents, changes to img element's attributes, changes to  
source elements' parents, and changes to source elements' attributes, and  
"await a stable state" and then process the picture element's img and  
source elements' attributes (for both the old and new picture element, if  
you moved an element between two picture elements), which is more complex  
than the attribute case.

-- 
Simon Pieters
Opera Software