[mmis1000]

Probably by

- locate the text node containing the selected text

- split the text node into 3 segment (the selected one in the middle)

- replace the selected segment with a span that has font style on it

?

I did something like this before to replace part of the text with a ruby tag.

https://codepen.io/mmis1000/pen/gOgKqba?editors=0010

Or probably you can do it the reverse the obfuscate the selected text and make it actually works like a black pen.

Although it will be definitely more complex because selection may cross multi text nodes. But the idea should apply.

[Vinnl]

Yeah, I was initially thinking block-level elements, but just selected text should be easier.