Browser canvas is one of the worst culprits: it has to keep a buffer with an uncompressed bitmap several screens in size.

Old browsers used to keep a single screen worth of canvas buffer, then redraw stuff as you scrolled... which made it a horrible experience. You can still find some of that with "clever" web designs where they replace fonts or move things dynamically as you scroll.

Then you have websites with "infinite scroll" that just keep increasing the canvas buffer size more and more and more, to infinity and beyond... and people wonder why their Facebook or Reddit tabs use so much RAM.