For Firefox:
See: "Bug 79021 - All stylesheets are loaded regardless of rendering media"
https://bugzilla.mozilla.org/show_bug.cgi?id=79021
[Open 5.5 years, no activity in past 3.5 years]
For IE:
Don't care - reporting bugs to MS is an exercise in pain and frustration,
involving having to provide your credit card details to do their QA for
them on their products (and then they say "sorry, we can't help" -
completely useless), or if you use the web feedback forms they just
ignore you for 9 years (at which point they discontinue the product)
even though it was clearly a 1-line trivial fix ... or at least these have
been *my* experiences reporting bugs to MS. And as the piece-de-resistance,
their IE bug database is now closed (
https://connect.microsoft.com/IE )
to both reads and writes. Nope - I'm leaving this one to a masochist.
At least this explains why we have half as many CSS loads as page views. :)