- From: Boris Zbarsky <bzbarsky@MIT.EDU>
- Date: Sun, 10 Jun 2012 14:24:43 -0400
- To: whatwg@lists.whatwg.org
On 6/10/12 12:51 PM, Scott Jehl wrote:
> I'd like to propose that the spec recommends that vendors defer the
> loading of Stylesheets referenced via inapplicable media so that they
> do not block page rendering.
One issue with this is that in many cases the UA may not yet know what
media queries apply to the document when starting the sheet loads. In
some cases it does, of course, but those may be rarer than you think...
> For example, if a stylesheet is referenced using a
> (min-width: 1000px) media query, a small-screen device could decide
> that this stylesheet should not be fetched at all because it could
> never apply on its screen dimensions.
Actually, it could, because the media query applies to viewport
dimensions (in CSS pixels), not screen dimensions. A trivial example
would be this document:
<!DOCTYPE html>
<iframe style="width: 2000px"
src="document-using-sheet-with-min-width-1000px"></iframe>
There are also various user actions that can similarly change the width
of even the toplevel window in CSS pixels such that it's larger than the
nominal device width in CSS px (e.g. zooming out, in UAs that change the
size of CSS px when you zoom).
> It merely re-evaluates if an inapplicable
> query would apply if it were a "device" query instead of a viewport
> query, and if not, the stylesheet is not loaded at all.
Except device queries and viewport queries are not at all the same
thing, of course. Conflating the two may be ok for a library that
developers are free to not use if it's making assumptions that are not
true in their use cases, but it's really not OK for a UA to do.
I agree that better UA behavior here would be nice, but it's not trivial
to do without breaking sites or creating a poor user experience.
> 2. Reducing the amount of time that an unresponsive stylesheet can
> block page rendering
Yes, indeed.
-Boris
Received on Sunday, 10 June 2012 18:25:19 UTC