http://lists.w3.org/Archives/Public/www-style/2013Jan/0434.html:
>
> avoid [or delay] the transfer of data that is not relevant to the
> state of the user agent at the time of the page load.

A possible proposal from Henri is an attribute on <link> that opts an
external style sheet out of synchronous CSSOM access, makes the sheet
not load and not delay the load event if its media query cannot ever
match in the UA even after zooming or invoking a print function, and
makes the sheet load with low priority and not delay the load event if
its media query does not match at the time the <link> element is
inserted into the document but might match later (e.g. if it's a print
style sheet, or depends on pixel density).