I currently make HTML5 mobile games for a living, and have generated $50,000 in revenue for myself and the content creators I work with over the past 11 months. My HTML5 games make most of their money from companies that wish to host the games on their websites and gaming portals, and they pay an upfront fee for the right to do so. HTML5 games have nothing on Flash when it comes to desktop, but mobile is where HTML5 is valuable. I'm working with companies that are paying $600+ for Tic Tac Toe games. Not even kidding.
Some of my games make money with ads, however there are only one or two companies doing this well and my endevors with them ar eonly just beginning.
You can check out my blog about HTML5 games and game dev in general at Touch To Start, and if you're interested in monetizing HTML5 games I have written an ebook called Making Money With HTML5 which has launched over a hundred aspiring developers into the market already.
Hope this helps!