Gerv pinged me yesterday night by email asking:

if and when we get browsers with native HTML overlay support, how do you prevent the backwards-compatibility JavaScript executing and thereby including two copies of the overlay contents?

Excellent question captain, thanks for asking. So I spent a few cycles on the problem yesterday. It's not an easy problem at all. The solution must be declarative, scriptless, simple, non-intrusive and of course degrade nicely in legacy browsers.

So I came up with this solution, posted on Gerv's blog (it's the 7th comment; unfortunately, Gerv's blog offers no permalink for comments). Update: found a better way in 11th comment on Gerv's blog.