Support CSSStyleSheet for .styles property

[samthor]

lit-element should support returning a CSSStyleSheet object from the static class getter .styles(), in order to support the upcoming CSS Modules spec.

This behavior should be supported:

import styles from './my-element.css';

class MyElement extends LitElement {
  static get styles() { return [styles]; }
}

Currently, I can do this, but it's using browser support only:

import styles from './my-element.css';

class MyElement extends LitElement {
  constructor() {
    super();

    // Lit doesn't clobber this, since it has zero styles
    this.shadowRoot.adoptedStyleSheets = [styles];
  }
}

[danielbarion]

Take a look: #747

Maybe this can help you...

[samthor]

@danielbarion that's not related. I don't have the raw string, CSS Modules gives you a parsed CSSStyleSheet.

[grvpanchal]

Can we have ability to modify LitElement.styles as we dynamically load styles?