[whatwg] defer on style, depends

Ian Hickson ian at hixie.ch
Mon Feb 9 00:23:37 PST 2009

On Sun, 8 Feb 2009, Jonas Sicking wrote:
> >>
> >> This will allow browsers to not block on those elements.
> >
> > Browsers are already allowed to not block on those elements.
> Well, Garrett is somewhat correct. For example in the following 
> scenario:
> <link rel=stylesheet href="external.css">
> <script>
>   doStuff();
> </script>
> In gecko, when we parse the <script> tag, we'll block until all external 
> stylesheets have finished loading before we start executing the script. 
> This is because the script might be getting .offsetLeft or calling 
> .getComputedStyle or some such, which uses style data. We have found 
> that some sites break if we just use whatever style data happens to have 
> loaded at that point, rather than ensuring that all stylesheets have 
> been parsed and applied.

Yes, external CSS can cause <script> elements to block on the CSS.

> However adding a 'defer' attribute to the style link seems like an odd 
> fix as nothing is actually deferred. You very rarely actually want to 
> defer stylesheet loading as you generally want to show content with 
> stylesheets applied. Unstyled content tends to at best be ugly, at worst 
> be totally nonsensical.
> A more logical fix would be to add an attribute to the <script> in 
> question, indicating that the script can be executed without waiting for 
> all stylesheets to load. Though that can be very painful to add to all 
> scripts in the page. Possibly a page-wide attribute or API would better 
> solve this problem. Though I'm not really a fan of either of those 
> solutions either.

I'm not convinced either of these are really great solutions. I think it'd 
be better just to have the script itself only block when it hits 
CSS-dependent APIs (though I recognise that that is a much harder problem 
in most rendering engines today).

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

More information about the whatwg mailing list