"Amaya is the name of W3C
's own browser
/authoring tool and is used to demonstrate and test many of the new developments in Web
protocols and data formats. Given the very fast moving nature of Web technology, Amaya has a central role to play. It is versatile and extensible - new features can be easily added - and is available on both Unix
and Windows '95
platforms." -- http://www.w3.org/Amaya/
It handles some MathML, but the really nifty point is that it will reliably generate valid HTML. You can give Amaya to someone who doesn't grok or even understand HTML, and the pages they create with Amaya will be suitable for many browsers. Thanks, W3C!