@Pesala , yes you can, but you can't upload the file (PDF or Html) in Vivaldi from the local drive, but you can open them from there with Vivaldi, which permits to add them to the reader list, at least in Windows.
Edit: Right, you can open them with Vivaldi and apears add them to the reader list in it's menu, but it don't work.
Maybe related that Vivaldi can't access direct to the local drive.
Confirmation dialogs may be useful at first, but if you use a feature frequently, they soon become a distraction.
If you use the Status Bar Overlay, the notification stays on screen until the page is reloaded, or the status field is rewritten by hovering a link, or whatever.
You are right about Reader View. Look the extension Reader View:
"Reader View extension brings Mozilla's open-source Readability"
Many many features that the built-in not have.
It has "Custom Styling":
And "Read this article":
The point is:
If you are going to implement a feature that many extensions execute study them, and do it equal or better or not do it.