>What is missing in HTML5?
- Audio (while in some respect capable) is a nightmare, the interface is unstable and breaks about once a year in my experience.
- You can't do pixel-perfect rendering (People keep promising that you can - the last time I checked was a few months ago - the DOM rendering architecture is deeply wretched when it comes to pixel-access).
- No nice pipeline for integrating vector animations/art into games like there was in flash. (SVG exists, and I've used it in games myself, but it's a niche thing to do). For most engines nowadays you're baking your SVG art to sprite, but flash kinda took care of it for you - its approach to vector art+animation was great/very very powerful, and Adobe Animate allows for HTML5 animation export nowadays, but that's somewhat short of a full interactive app pipeline.
- many of the html runtimes for new engines (Unity for instance) feel waay heavier/more sluggish in the browser than flash games. Pauses during loading/weird interactions with the mouse, having to click before you can see anything is common behaviour. And often they don't perform so well as the flash runtime.
- easy to make fancy + good-looking interactive UIs. There's a reason Scaleform was so beloved for UIs even outside of the browser ( https://en.wikipedia.org/wiki/Scaleform_GFx ).
It's not obviously better than flash still. Flash was a very stable platform for many many years - really good with backwards compatibility (IIRC as a result of literally having secretly several copies of the runtime for different versions), and ...well, for instance Godot HTML5 builds as of last week crashes in safari for everyone it seems.
Source: I made bunch of flash games, and have made a bunch of HTML5 games, and have made a HTML5 game engine myself. And flash was just a better experience as a user for so many things (except for abuses) and as a developer. Outside of writing javascript yourself, haxe is probably the best replacement for flash (as well as being good in its own right). I have been playing with heaps this week actually and it's yeah pretty nice. But flash had a nicer pipeline.
Flash was weaker at 3d stuff, though - even back when flash was a thing I still went for Unity/hand-rolled for 3d games.