We do a lot of rich internet apps which are heavy HTML/JS and get asked a lot "Are you using HTML5" and I'd go down this whole spiel about "well really we like the CSS3 features" and "a lot of what we do isn't HTML5 its just HTML4 and Javascript" or "Maybe we try to use local storage but have to fall back to standard cookies, and backend persistence for legacy browsers".
Eventually gave up and now when people ask if an app is "HTML5" I just say "yes".
My self-imposed rule is if the page's doctype is <!DOCTYPE html> then I say it's HTML5 (regardless of what other technologies I actually use - it could just be a static page for all I care).
Eventually gave up and now when people ask if an app is "HTML5" I just say "yes".