That's a really interesting question and I considered it for half-an-hour.
I cannot say I have a way of finding out. certainly most should have now enabled use because we were discussing adoption back in 2000 but as to find out which ones I don't know...apart from the obvious setting up an Apache box. All the new browsers should have it enabled.
I will think about it over the weekend. Apologies for not being able to point in the right direction yet, though I pretty much guess your resources are the same as mine.