When I decided that I needed to learn some MathML I looked around for a suitable tutorial. Unfortunatley I couldn't find any of use, so I turned to the specification and gradually worked my way through it. So that you don't have to, I made a tutorial at the same time:
Read the MathML Beginner's Guide
It is, and I guess should always be, a work in progress so I'm adding to it all the time.
My next plan for a tutorial article is something where there is also a shortage of clear addvice online (though this is understandable since the specification is still in flux) : the CSS3 border-image property. There's only a place-holder page there at the minute, but I will build on it soon.