<!DOCTYPE html>
<style>
    #target::before { content: "FAIL (string) "; }
</style>
<div id="target" my-attr="FAIL (attribute) ">test</div>
<script>
onload = function(){
    var s = document.createElement("style");
    s.textContent = "#target::before { content: attr(my-attr); }"
    var head = document.querySelector("head");
    head.appendChild(s);
    target.offsetTop;
    target.setAttribute("my-attr", "PASS ");
};
</script>