Check if Div with specific Class exists in the page

Question

I\'m trying to modify an existing webpage by inserting my own Javascript file in it. I\'m setting a background Image for this webpage by inserting a new Div through my Javascrip

Answer 1

document.addEventListener("DOMContentLoaded", function() {
  var ie = document.getElementsByClassName("ie");
  if(ie.length > 0) {
    alert("It's IE!");
  }
}

This should answer your first question, although I agree that there are better solutions for your overall goal.