JSFiddle - React, Tailwind, and code Playground

HTML

<button class="special" id="pressme">Press me</button>

<p>CSS rule 'button.special' found: <b id="cssbp"></b></p>
<p>JS object 'window.location' found: <b id="jswin"></b></p>
<p>JS method 'CSSRuleExists' found in window object: <b id="jscrl"></b></p>
<hr />
<p>CSS rule 'button.cool' found: <b id="cssbc"></b></p>
<p>JS object 'epicfail' object' found: <b id="jsef"></b></p>
<p>JS method 'Imnothere' found in window object: <b id="jsnef"></b></p>

CSS

button.special {
    background: #eee;
    border: 1px solid #555;
    border-width: 1px 3px 3px 1px;
    transition: all .2s;
    padding: 8px;
    outline: none;
}

button.special:active {
    border-width: 3px 1px 1px 3px;
    background: #e9e9e9;
}

JavaScript

window.CSSRuleExists = function(rule) {
    var sSheetList = document.styleSheets;
    for (var sSheet = 0; sSheet < sSheetList.length; sSheet++) {
        var ruleList = sSheetList[sSheet].cssRules;
        for (var item = 0; item < ruleList.length; item ++){
            if(ruleList[item].selectorText === rule) {
                return true;
            }
        }
    }
    return false;
}

window.JSFunctionExists = function(method, parent) {
    parent = (typeof parent === typeof undefined) ? window : parent;
    return (typeof parent[method] === 'function');
}

window.JSObjectExists = function(object, parent) {
    parent = (typeof parent === typeof undefined) ? window : parent;
    return (typeof parent[object] === 'object');
}

function process() {
    document.getElementById('cssbp')
            .innerHTML = CSSRuleExists('button.special');
    document.getElementById('jscrl')
            .innerHTML = JSFunctionExists('CSSRuleExists');
    document.getElementById('jswin')
            .innerHTML = JSObjectExists('location');
    document.getElementById('cssbc')
            .innerHTML = CSSRuleExists('button.cool');
    document.getElementById('jsnef')
            .innerHTML = JSFunctionExists('Imnothere');
    document.getElementById('jsef')
            .innerHTML = JSObjectExists('epicfail');
}

document.getElementById('pressme')
        .addEventListener("click", process);