As @briankardell knows well, I am very supportive of this idea
I'm happy to set up a Paris chapter (and certainly happy for help with it).
@tobie I agree that we shouldn't be overly optimistic about it and we should start with simple expectations. Already having a place for Web developers who care about standards to socialise is a good first step: that's how a lot of stuff got started.
Then as we start building extra infrastructure, people can start contributing progressively. Keep in mind that TestTWF suffers from a limitation that this idea doesn't: here we are talking about local user groups that could for instance meet every months. If they did that for testing, the quality and commitment would certainly increase over time! In fact, Chapters could take responsibility for local TestTWF/WPT hacking.