[whatwg] Add an attribute for opting out of synchronous CSSOM access

[Henri Sivonen]

For problem statement, please see
http://lists.w3.org/Archives/Public/www-style/2013Jan/0434.html

For solution, please see
http://lists.w3.org/Archives/Public/www-style/2013Jan/0457.html

For CSS WG thinking that this is an HTML issue, please see
http://lists.w3.org/Archives/Public/www-style/2013Mar/0688.html
(FWIW, I think this is a CSS issue than requires an HTML attribute to
be minted.)

Please add an attribute to <link> that:
 * opts an external style sheet out of synchronous CSSOM access
 * makes the sheet not load and not defer the load event if its media
query cannot match in the UA even after zooming or invoking a print
function
 * makes the sheet load with low priority and not defer 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).

--
Henri Sivonen
hsivonen at iki.fi
http://hsivonen.iki.fi/