[cssom] Resolved value for font-size should perhaps take user's minimum font-size into account

[dholbert]

CSSOM section 9 on resolved values (i.e. the value returned by getComputedStyle) doesn't have any special case for the font-size property right now.

However: users can configure a minimum font-size to enforce in their browser settings; and in Safari and Chrome, this clamping does seem to be reflected in the values that getComputedStyle(elem).fontSize returns. In Firefox, the clamping doesn't take effect in getComputedStyle and we got a bug report on Firefox differing from other browsers in this respect: https://bugzilla.mozilla.org/show_bug.cgi?id=1903360

Perhaps we should spec this clamping behavior as part of the resolved value for font-size? (and explicitly apply the clamping in the getComputedStyle implementation) e.g. just specify that the resolved value of font-size is the used value, or something along those lines? (I'm not sure if that would bring in any additional complexity...)

Additional notes:

• I've got a testcase to poke at some edge cases here: https://bugzilla.mozilla.org/attachment.cgi?id=9408768
• Safari behaves a bit differently from Chrome/Firefox in that they seem to let the font-size impact the resolved value of 1em, not just the size of rendered text. That's probably a Safari bug (CC @heycam / @nt1m ).
• https://bugzilla.mozilla.org/show_bug.cgi?id=1903360 includes instructions for how to find the relevant preferences UI in Firefox, Chrome, and Safari.

[svgeesus]

Adding myself just to track this @svgeesus