My most used bookmark is this, for speeding up the first audio or video element on the page. I suppose that's if you don't count me typing 'n' and auto-completing my way to hacker news.
javascript:(function() { (document.querySelector('video') || document.querySelector('audio')).playbackRate = (+prompt('How many times normal speed?') || 1); })();
On YouTube I've had a great experience using Sponsorblock which crowd sources categorizing different parts of the video. If a bunch of people categorized the first 10 seconds of a particular video as "Intro", that section of the video scrub/seek bar is highlighted in a color that represents that category. I have Sponsorblock set to skip Intros, so I never see them unless it's a very very fresh video that nobody has categorized yet.
It's baked into YouTube Vanced on mobile, which was sadly taken down by Google. Smart Tube Next is another Android app with this feature, but I'm not sure if it's available for anything other than TV set top boxes.
