Well, that's one possibility. I am imagining instead a mode that tells the user-agent "Use your own standard HTML stylesheet, and it's allowed to change and be

Well, that's one possibility. I am imagining instead a mode that tells the user-agent "Use your own standard HTML stylesheet, and it's allowed to change and be updated over the years, to fix bugs and improve design, you don't have to stick with your default styles from 30 years ago,' but "use the same stylesheet you are using for other pages in this mode, I don't want to have to come up with it, and I don't want it to be fixed in time to what I come up with today either or require my maintenance" That is, i suppose, more like "reader/readability mode" in all those aspects--but triggered by the source instead of by the user. But, sure, a CSS stylesheet is another thing. I'm just thinking around the use cases I think OP is setting out; you can do so too, with other suggestions, you don't have to try telling me I mean something other than I mean to do so!