[css-size-adjust] Specifying text-size-adjust: <percentage>

The text-size-adjust CSS keyword has a prefixed implementation in most
major engines today (an exception being chromium) which lets authors
control how adjustment works. The <percentage> value is currently
underspecified and I'd like to spec it to match both Trident and WebKit.

Ideally, we'd remove <percentage> but the httparchive data shows percentage
values are pretty common (1.2% of all files [1]). Because the
implementation is not too onerous and supporting percentages with ease the
transition to unprefixed properties, I'd like to spec it instead of remove
it.

I'd like to define <percentage> as a set value that gets multiplied by the
specified font size. Percentage values are not relative to any other
adjustment and text-size-adjust: 100% is equivalent to text-size-adjust:
none. Trident and WebKit appear use this logic [2] and it is compatible
with existing pages and MS/MDN documentation [3,4]. For posterity, Gecko
does not support <percentage> and Chromium does not yet implement
text-size-adjust at all.

I have a preview of this change at:
https://github.com/progers/csswg-drafts/commit/1f80533bc0f5eb8e97fbf4ae113af5f731756140
And a preview of the updated spec at:
https://rawgit.com/progers/csswg-drafts/master/css-size-adjust/Overview.html#adjustment-control

[1] Example: http://www.pne.ca/css/iphone.css
[2]
http://trac.webkit.org/browser/trunk/Source/WebCore/css/StyleResolver.cpp?rev=194514#L2069
[3] https://msdn.microsoft.com/en-us/library/dn793579(v=vs.85).aspx
[4] https://developer.mozilla.org/en-US/docs/Web/CSS/text-size-adjust