Constructible style sheets polyfill

This package is a polyfill for the constructible style sheets/adopted style sheets specification. The full specificaiton is enabled by default in Google Chrome as of version 73.

Currently Mozilla is considering implementation of the feature, marking it as "worth prototyping" while Apple has not publically signaled, they have been active in the standards discussions surrounding it.

Use case

The constructible style sheets proposal is intended to allow for the dynamic creation and sharing of style sheets, even across shadow boundaries. By adopting a style sheet into a shadow root, the same sheet can be applied to multiple nodes, including the document.

How it works

This polyfill will create a new style element for every DocumentOrShadowRoot into which the sheet is adopted. This is counter to the current proposal, but updates to the style sheet using the replace or replaceSync methods should update the relevant style elements with the updated content across all adopters.

No changes will occur in a browser that supports the feature by default.

Installation

The package can be installed either by copying the polyfill file or by installing using npm.

Usage

const everythingTomato = new CSSStyleSheet();
everythingTomato.replace(`
* {
    color: tomato;
}
`).then(console.log); // will log the CSSStyleSheet object

document.adoptedStyleSheets = [everythingTomato];

class TestEl extends HTMLElement {
    constructor() {
        super();
        this.attachShadow({ mode: 'open' });
        this.shadowRoot.adoptedStyleSheets = [everythingTomato];
    }

    connectedCallback() {
        this.shadowRoot.innerHTML = `<h1>This will be tomato colored, too</h1>`;
    }
}

customElements('test-el', TestEl);

const testEl = new TestEl;
document.body.appendChild(testEl);

The polyfill will append new style tags to the designated DocumentOrShadowRoot. Manually removing the style node will cause a re-insertion of the styles at the designated root. To remove a style sheet, you must remove the style element from the element.adoptedStyleSheets array. The behavior here is supposed to emulate a FrozenArray, so modifying the array in question will have no effect until the value is changed using a setter.