JSFiddle - React, Tailwind, and code Playground

HTML

<pre id="console"></pre>

JavaScript

var rulesForCssText = function (styleContent) {
    var doc = document.implementation.createHTMLDocument(""),
        styleElement = document.createElement("style");
    
    styleElement.textContent = styleContent;
    // the style will only be parsed once it is added to a document
    doc.body.appendChild(styleElement);
    
    return styleElement.sheet.cssRules;
};

var rules = rulesForCssText("span { font-size: 12pt; }"),
    fontSize = rules[0].style.fontSize;

document.getElementById("console").textContent = "Your span's font-size is " + fontSize;