I am, too. The same font rendered by Core Text, FreeType, and Direct2D/DirectWrite will not have the exact same dimensions every time despite coming from the same exact file.

Say I render a font with no anti-aliasing on an embedded device, because it's marginally more expensive to do so. If I did anti-alias it, you have to account for possibly growing the bitmap size to do so. If I did, am I no longer compliant with web standards? No, because the web standards don't specify dimension requirements for text nodes from source fonts.

Further, there are multiple different types of ways to anti-alias. Should the web standards dictate how?