With advocates like that, who needs enemies?

Browsers 'should' do things, or are downright broken, in many different ways, most of which we spend a lot of time hacking around to fix. I don't think accessibility related issues should be any different.

Yes, content should be viewable, but to whom? Just able, sighted users. Not for our colour-blind users? In this sense, viewable's an elastic word 

Basically what I'm getting at is the fact that yes, there are limits to how user-inclusive we can make the site, but with this case in particular, I've never found it to be too much of a headache to use `em`, seeing as fonts shouldn't get that nested, and sizes rarely change. If it allows us to solve an issue (which may or may not be an issue), and has a relatively small down-side.