As one of my summer projects I'd like to build an AJAX program on my own. Not too big. I'm new to the whole matter, however I have general knowledge in programming and a good understanding of the Web workflow.

As for W3C standards, is it wrong sticking to them as if they were a *platform*?

Is it a good idea to code having a single platform in mind (i.e. Firefox 3 + Win7) and make it cross-browser *after the initial code is finished*?

In general, to what extent is browser compatibility a problem these days?