> should I write "a fo:block" or "an fo:block"?
> I am not a native speaker, so this is not clear for me. I think it 
> depends on whether I spell "fo" or read it as a word.
> 

The rule is, as you suggest, that it depends on pronunciation not on
spelling. Thus "an XML document" but "a Xerces error message".

I think most people pronounce "fo:block" as "eff-oh-block", but I may be
wrong!

SQL causes havoc, because half the world pronounces it "ess-cue-ell" and the
other half "sequel".

Michael Kay (not a native speaker either - well not of English, anyway!)
http://www.saxonica.com/