JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
  <li id="myli" class="myclass">Hello</li>
</ul>

CSS

.myclass{
   color:red;       
}
li.myclass{
   background-color:black;
}
ul li{
   float:left;
}

JavaScript

function css(a) {
    var sheets = document.styleSheets, o = [];
    for (var i in sheets) {
        var rules = sheets[i].rules || sheets[i].cssRules;
        for (var r in rules) {
            if (a.is(rules[r].selectorText)) {
                o.push(rules[r].selectorText);
            }
        }
    }
    return o;
}

console.log(css($('#myli')));