I expect one reason why the html police isn't going after the font size is that it isn't set in px, but in pt, so the browser scales the font differently depending on the screen's dpi.
Setting the font size in em's sounds utterly ludicrous to me, as the em is a unit sized relative to the current font size (normally the width of a lowercase "m").
Reductionist and proud of it.
Being ignorant is not so much a shame, as being unwilling to learn. Benjamin Franklin
Chase after the truth like all hell and you'll free yourself, even though you never touch its coat tails. Clarence Darrow
A person who won't read has no advantage over one who can't read. Mark Twain