Computer Help and Support

You can use a Reader extension.
It alters the formatting like crazy (on purpose) but in doing so it almost always makes text copy-able (No doubt by killing the javascript that does so)
Another way is to turn off javascript for that site or page. But that might prevent text at all if javascript creates the text.
And yet another is to save/print to pdf.

You can then share, or even mark up the PDF.
Yes, it takes expensive apps, including Adobe stuff, to do this is doable to various extents.

I am familiar with ones in ipad/ios, like Notability, NoteShelf, MarginNotes and others. (not lately though)

Given that there are few or no linux tablets, I have been OK with Apple ipads.
I got an ipad 1, and still use it to display music scores. (and books)
That saves about 30 pounds or more of printed scores.

In fact, there are cool things like a-Shell (a complete linux VM) Carnets (Jupyter Notebooks, batteries included), the Processing Language, Pythonista, LuaStudio and so on. All free, IIRC.