HTML+JS+CSS is such a crappy alternative for animation.
Looking forward, it'll be incredibly hard to archive our "modern" web animations. Everyone does it differently, and the results are spread across dozens of files and a hodgepodge of libraries. Flash is self-contained in a single file and the platform has everything you need baked in. Easy.
You didn't even need a website for flash. You could download the swf and play it locally, like a movie file or gif. And you could even share them with friends. It was amazing.
The only reason we don't have Flash is because of Steve Jobs. He killed it out of greed so he could gatekeep his platform.