HowTos give useful advice on how to use an open source project. Many projects have links to HowTos directly, and this page aggregates all content tagged “HowTo”.
Focuses on the XML+XSLT->XHTML case, where the final HTML layout comes from the XSLT, basically just picking variables from the XML input. This just is never true for me; my input tends to be XHTML too, and most of the transforms I do try to keep that input as unedited as possible.