I'd rather avoid captchas altogether, they just annoy people while doing little to actually prevent spammers.
My proposal is to go for some user-configurable Q&A,. Then again, good luck educating admins to bother setting it up properly so it's both
accessible enough for users and not too easy for spammers. (
I remember setting up a phpbb board over 4 years ago that I've long since left, and there was a 'sortable q&a' feature that had minimal JS and
made you put different things into different boxes:
I think there's maybe been one spam account that registered in the whole time of its existence, and while it's... obnoxiously opinionated, to say the
least, it works for the purpose of that specific forum.
In other words, configure something like this correctly, and you've got yourself something that works with minimal JS and tracking things - therefore
works even under 'strained conditions'. You could expand on the idea a bit more than phpbb did which I think only allows for the one question - as
long as the suggested defaults are strong I think admins should be okay
[insert witty quote here]