Reading through this, I am excited about the idea of being able to select some text and apply <mark></mark> code to it so it is highlighted. And also the ability to choose a highlight color or treatment. I often do this to draw attention to a key sentence in my posts. I also use <kbd></kbd> to refer to buttons in my posts.
But the place where I want to do this is in the composer when writing my post, not when I am reading posts. The quote/fast edit/share popup makes sense for those three actions but I don’t really see the point of adding a highlighter button there.
And why would each user have their own highlighter color? What’s the use case for that?