New PDF editing tools are cool and extremely useful. Some of them would be incredibly useful on standard HTML websites as well.
For example, one could research an article on their laptop, add some annotations on the most important information, mark the place they paused reading and pick up their work on desktop with Firefox Sync. Even most basic functionality, such as text highlighting and adding notes to it, would be extremely useful for many people. Imagine working on a project and having to look up something again – with this functionality you could simply open the website and jump to the note on the topic you marked previously, immediately, even after few months, without having to read through entire content again.
A similar idea has been proposed, but it focuses on manual notes overlaying the website, without ability to pin them to content, sync or recover later. Such feature would be more useful for people presenting their screen to other in video calls, as the author points out. https://connect.mozilla.org/t5/ideas/annotate-websites/idi-p/22505