Scratch (I'd suggest turbowarp specifically as you can export it to html5) is a really simple way to make animations in my experience. (Haven't made anything too complex though, but still pretty decent.)

Slight spoilers for the adventure if you look at them but here's a few examples:

https://mspfa.com/?s=53466&p=9

https://mspfa.com/?s=53466&p=10

https://mspfa.com/?s=53466&p=17

There's the added benefit also of it being a full block-based coding platform, so you can if you want make "flash games" with it