Good point -- I shouldn't assume that everyone's using an editor that does syntax highlighting and validation.

Some of the browsers have syntax coloring builtin when you view source, and there are a few that have built-in validators (iCab comes to mind). I've personally reset my source viewer with my text editor (BBEdit, as I'm a mac user). For those who haven't been writing HTML for more than a decade (damned <br> tag -- should've just made <address> preserve line endings)

Anyway, there is a validation add-on for Mozilla, and there are even some validators in CPAN, like HTML::Validator and HTML::Tidy