the browser doesn't survive playing more than a few of them reliably at the same time
In contrast, around the turn of the century there were plenty of webpages stuffed full of annoying animated Flash ads, which as irritating as they were, managed to work well enough on the typical single-core CPU of the time.
Even in the mid-2000s, when most PCs had 512MB - 1GB RAM, you could build complex 3D animations in Flash that ran flawlessly across multiple browser engines including IE6, with nothing more than a plugin.
Don't know why, but I take great pleasure in saying something was something I did "last century" when talking about software or web design.
It varied a fair bit. One of the reasons I started using adblocking measures was because flash ads would pretty often get my single core PPC Mac’s fans spun up and angry.