The content from the web site says it far better than I will, but it looks like jsMath was inspired by the slow adoption of MathML support in browsers running under Windows, Mac, and *nix machines.
The examples that the web site offers look beautiful. It's based on TeX, so it's no surprise that the results look so good. I didn't get a chance to dive into it this weekend. Unseasonably nice weather on the East Coast made it possible for me to clean up all the leaves that were covering my yard, so time to program was hard to come by. But I'll be looking into this gem soon. It's a nice complement to my recent rediscovery of LaTeX.