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.

Here's the fork that added Firefox support (https://github.com/codebicycle/videospeed, https://addons.mozilla.org/en-US/firefox/addon/videospeed/), and it's saved me so much time it's absolutely ridiculous at this point.

But a bookmarklet saves you from having to run an extension at all times.