You forget that webpages are allowed to do what ever the hell they want, and so each is probably running a script or two in the background. We put up with a lot in webapps that would never stand in a desktop app.
Firefox is working on this. Doing it without breaking things like music streaming sites is ... delicate.
Earlier this year I realized a major offender was Google search and search results pages which would spin up a core more than once a minute.
This explained to me at least why it was still hogging cpu even with ad blocker.
https://billmccloskey.wordpress.com/2016/10/27/mozillas-quan...
and a preliminary version is in Chrome 56: