Browser Demoscene
These are some cool websites I've found that I think are really worth visiting at least once just for the experience. Calling them "websites" almost feels wrong; they're interactive art experiences that happen to use the browser as a medium.
corru.observer
This is a surprisingly deep and multi-modal visual novel about an alien species that came to Earth and tried to befriend humanity. Then some weird stuff happened. I won't spoil any more than that.
lyra.horse/antonymph
This is a music video that uses your browser as the canvas. It may not be your thing if you're not a fan of pony music and don't really care about web development, but if you like even one of those things it's probably going to be a pretty fun time.