"The solution is arcane but I promise it works:"
//div[contains(concat(' ',normalize-space(@class),' '),' foo ')]
"Note that there must be spaces on either side of the class name 'foo'."
That's pretty long and is more general than I need. If multi-class entries are normalized already (e.g., no non-space whitespace in there, just spaces), then it's simpler to use just:
That's so much shorter that I generally start with that and only fallback to the longer form if I find any entries in the html that have non-space whitespace.