since it runs through the Chrome browser, could this also mean you could run HTML5 games directly?
Well, you can already run HTML5 games directly in the browser!
I don't think Stadia support makes much sense for HTML5 games - it seems best suited for running heavyweight games like Assassin's Creed that typically require a beefy gaming PC on a lightweight device like a Chromebook. Construct games generally can already run well on lightweight devices. So there doesn't seem to be any technical benefit to it.