JSFiddle - React, Tailwind, and code Playground

HTML

<div class="hello-world"></div>
<pre></pre>

CSS

.hello-world {
  display: inline-block;
  border-radius: 50%;
  background: blue;
}

.hello-world {}

.hello-world {
  width: 50px;
  height: 50px;
}

.hello-world {
  border: 3px solid red;
}

JavaScript

const getClassStyles = className =>
  [...document.styleSheets].reduce((styles, { cssRules }) => {
    return [...cssRules]
      .filter(n => n.selectorText === `.${className}`)
      .reduce((styles, { cssText }) => {
        return (cssText
          .match(/[\w\-]+: [^;]+/g) || [])
          .map(n => n.split(': '))
          .reduce((styles, [ k, v ]) => (styles[k] = v, styles), styles);
      }, styles);
  }, {});


const styles = getClassStyles('hello-world');
document.querySelector('pre').textContent = JSON.stringify(styles, null, 2);