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I've also come across this one before, and as M0nty says, it's 99.99% unenforcable.
Like most legislation in the UK at the moment, it seems to be dreamt up by people trying to justify their job rather than to try and address any real issues - rant over.
However, it's more to do with accessability/readability standards than followin correct W3C guidlines.
If you want to try to get around it then create a text only theme with large fonts, give it a name something like "accessability" and stick it in the theme switcher. Nobody could possibly complain then (OK - Nobody *should* be able to complain about it then). If you don't want the theme switcher block visible to everyone, then create an "Accessability" link off the main page to a page with just the theme switcher on it, and maybe some explanation.
Don't follow me, I'm lost too!