CSS

Source file extensions: .css

Import from Javascript/Typescript

Import globally 

import './style.css';
import 'https://web.site/style.css';

This import will transform the CSS file into an ES Module, the CSS is then put inside a <style> tag in the <head> of the page, applying the styles globally to the page.

Note that these global CSS imports will not be put inside the component bundle as it is considered a "side effect" of this JavaScript file. All globally imported CSS files are put into a style.css file in the root of the package. See also how to consume your design system.

Import as a CSSStyleSheet 

import style from './style.css';
import style2 from 'https://web.site/style2.css';

This method will create a CSSStyleSheet for use with adoptedStylesheets, which can also be used in the document (document.adoptedStyleSheets) or in ShadowDOM (node.shadowRoot.adoptedStyleSheets).

See Constructible StyleSheets for more information.

It is also possible to import the CSS as a raw string if you need to handle it in a different manner:

import style from './style.css?raw'; // CSS as string

Import as a CSS Module

If the imported file ends with .module.css, then it's imported as a CSS Module.

/* index.js */
import styles from `./style.module.css`;
...
element.innerHTML = '<div class="' + styles.className + '">';

/* style.module.css */
.className {
  color: green;
}

References

Quick example

style.css

strong {
  color: red;
}

div.menu-bar li:hover > ul {
  display: block;
}