title: CSS selector with nesting square brackets react
body: <p>I came across a CSS selector with nesting square brackets like this in a library. Tried to search and understand what does it mean, but can't find any information. Can anyone explain it:</p>
  <pre class="lang-css prettyprint-override"><code>[['h1', 'h2', 'h3', 'h4', 'h5', 'h6']]: {
      // CSS props here
  }
  </code></pre>
  <p>Edit: it's in the <code>style</code> props of GlobalTheme component from emotion lib, the project using React.</p>