Why is document.querySelector not working on pseudo element?

Answer 1

If you want to style pseudo elements from javascript you have to use the CSSOM to inject the rules. It's not trivial, but it's possible.

var sheet = document.styleSheets[0]; //get style sheet somehow
var rules = sheet.rules; 
sheet.insertRule('div:before { display: none; }', rules.length);

a slightly modified example from this article

CCSOM Reference