To be precise, they support MathML Core, which is a subset of MathML 4. Your expressions that you wrote for MathML 3 and worked in Firefox may not (and probably will not) work in Chrome. I actually wrote a library a little while ago which doesn’t work in Chrome because of this.

That said, I think MathML Core is an excellent standard, and Chrome made the right choice to support it (or rather to allow Igalia to develop it for them). Going forward I’m certainly going to update my library to target MathML Core only.