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CSSOM needs to define how Link:, <?xml-stylesheet?>, <link rel=stylesheet>, and <style> interact with the fetching algorithm, the event loop, and the parsers from HTML5.
<?xml-stylesheet?> could do with a rewrite for integration with CSSOM; do 'load' and 'error' events fire on it?
CSSOM should have a mechanism for taking elements full-screen
it has been proposed that CSSOM have a mechanism for keeping track of when expensive-to-compute areas of the document (e.g. a canvas) are actually being rendered.
Transition scenarios the CSSOM has to cope with somehow:
Property changes into a longhand property. E.g. some css3 text stuff..? Sometimes this only partially happens. E.g. with 'overflow' the situation is weird iirc.
Property value changes into a comma-separated list. E.g. background-image: <uri> becomes background-image: <uri> [, <uri>]*.
Property value changes into accepting multiple component values. E.g. the 'cursor' property now takes <x> and <y> in addition to <uri>. 'overflow' now takes two values rather than one.
Property component value changes. E.g. <uri> becomes <image>.