I had heard rumors (partly via Jerry Maddox) that you could actually post the same flavor of MathML on Firefox, Internet Explorer and Safari, and it would actually WORK.
I was able to do just that today. You can see the HTML5+MathML page as proof. More details are on the updated MathML Tutorial page.
Of course you can also check out this detailed MathML test page from Josephus “Joe Java” Javawaski. Safari is still a little behind the curve there.