Could you be more specific about the meaning here of "pure HTML"? Just markup and content? We used to call that the World Wide Web 😕

Does it have a reading pane or will it stretch the width of my monitor? If it limits the reading width, then OG.

Well see there was this thing in CSS called user style sheets that you could use to write your own styles that would override those provided specific to your needs. You can still do it. Why don't we know about it probably as the primary use case was largely for accessibility.

I’ve dragged my user.css file from computer to computer over the passed 20 years.