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

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 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.