I'm not being flip about it. Granted, entropy is an issue with in-browser JS, but the rest of it is really just cautionary about browser bugs, compromised browsers, and known page-based browser attacks. Very little doesn't also apply to every crypto system on every platform. If you can't trust your OS or browser, you've got bigger problems.

I'll submit that because it's in-browser it's additionally difficult - very difficult - to get right, but I don't see that it's impossible.