HTML Imports

Learn the tech

Why HTML Imports?

This webcomponents.js repository contains a JavaScript polyfill for the HTML Imports specification.

HTML Imports are a way to include and reuse HTML documents in other HTML documents. As <script> tags let authors include external JavaScript in their pages, imports let authors load full HTML resources. In particular, imports let authors include Custom Element definitions from external URLs.

Basic usage

For HTML imports use the import relation on a standard <link> tag, for example:

<link rel="import" href="import-file.html">

Polyfill details

Getting Started

First download the webcomponents.js polyfills using Bower:

bower install --save webcomponentsjs

Include the HTMLImports.js or HTMLImports.min.js (minified) file in your project.

<script src="bower_components/webcomponentsjs/HTMLImports.js"></script>

Alternatively, you can directly use webcomponents.js (or the minified webcomponents.min.js file) in your project.

Polyfill Notes

In imported documents, href and src attributes in HTML, and url properties in CSS files, are relative to the location of the imported document, not the main document.

The HTML Imports polyfill begins processing link tags when the DOMContentLoaded event fires. To know when loading is complete, listen for the HTMLImportsLoaded event on document or window.

Example:

<script>
window.addEventListener('HTMLImportsLoaded', function(e) {
  // all imports loaded
});
</script>

The polyfill loads linked stylesheets, external scripts, and nested HTML imports, but does not parse any data in the loaded resources. For parsing imports, combine HTML Imports with Custom Elements. As long as the HTML Imports is loaded first, the Custom Elements polyfill will detect it, and process all imports when HTMLImportsLoaded event fires.

The WebComponentsReady event

Under native imports, <script> tags in the main document block the loading of imports. This is to ensure the imports have loaded and any registered elements in them have been upgraded. This native behavior is difficult to polyfill so the HTML Imports polyfill doesn't try. Instead the WebComponentsReady event is a stand in for this behavior:

<script>
  window.addEventListener('WebComponentsReady', function(e) {
    // imports are loaded and elements have been registered
  });
</script>

Other notes

  • In a native HTML Imports, document.currentScript.ownerDocument references the import document itself. In the polyfill use document._currentScript.ownerDocument (note the underscore).

Tools & Testing

For running tests or building minified files, consult the Manual Builds guide.

Polyfills