What does this look like and why do it? We’ll get to that in a sec. First, remember how HTML and CSS normally work. A class is applied in HTML:

<h1 class="title">An example heading</h1>

And that class is styled in CSS:

.title {
  background-color: red;
}

As long as that CSS is applied to the HTML document, the background of that <h1> would be red. We don’t need to process the CSS or the HTML. The browser understands both those file formats.

Source: What are CSS Modules and why do we need them? | CSS-Tricks