Microtransactions are the big income source for web games, income from ads tends to be fairly minor unless you can get millions of displays.
Most smaller games are made because someone wanted to make them, not because someone wanted to make money.
Microtransactions? What do you mean?
If you're saying you don't know what microtransactions are: It's a common source of money for game makers these days. I recommend you try Googling the term. http://lmgtfy.com/?q=microtransactions
If you're saying you don't know how microtransactions apply to "flash/HTML5 games," then that's a different question.