HTML Imports polyfill
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Latest commit a83cc70 Jun 12, 2018


A polyfill for HTMLImports.

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The polyfill hosts the imported documents in the import link element. E.g.

<link rel="import" href="my-element.html">

<!-- becomes -->

<link rel="import" href="my-element.html">
  <!-- my-element.html contents -->

The polyfill fires the HTMLImportsLoaded event when imports are loaded, and exposes the HTMLImports.whenReady method. This api is necessary because unlike the native implementation, script elements do not force imports to resolve. Instead, users should wrap code in either an HTMLImportsLoaded handler or after load time in an HTMLImports.whenReady(callback) call.

The polyfill provides the HMTLImports.importForElement() method which can be used to retrieve the <link rel=import> that imported an element.

Caveats / Limitations

<link>.import is not a Document

The polyfill appends the imported contents to the <link> itself to leverage the native implementation of Custom Elements, which expects scripts upgrading the CustomElementRegistry to be connected to the main document.

As a consequence, .ownerDocument will be the main document, while .parentNode of the imported children will be the <link rel=import> itself. Consider using HMTLImports.importForElement() in these cases. e.g:

const ownerDoc = HTMLImports.importForElement(document.currentScript);
let someElement = ownerDoc.querySelector('some-element');
if (ownerDoc !== HTMLImports.importForElement(someElement)) {
  // This element is contained in another import, skip.
  someElement = null;

If you require document isolation, use html-imports#v0.

Dynamic imports

The polyfill supports dynamically added imports by observing mutations in <head> and within other imports; it won't detect imports appended in <body>.

If you require to append imports in <body>, notify the polyfill of these additions using the method HTMLImports.loadImports(document.body).

Imported stylesheets in IE/Edge

In IE/Edge, appending <link rel=stylesheet> in a node that is not <head> breaks the cascading order; the polyfill checks if an import contains a <link rel=stylesheet>, and moves all the imported <link rel=stylesheet> and <style> to <head>. It drops a placeholder element in their original place and assigns a reference to the applied element, placeholder.__appliedElement. e.g.

my-element.html imports a stylesheet and applies a style:

<link rel="stylesheet" href="my-linked-style.css">
<style> .blue { color: blue }; </style>

And is imported in index.html:

  <link rel="import" href="my-element.html">

This is how the resolved import will look like:

  <link rel="stylesheet" href="my-linked-style.css">
  <style> .blue { color: blue }; </style>
  <link rel="import" href="my-element.html">
    <link type="import-placeholder">
    <style type="import-placeholder"></style>

The placeholders contain a reference to the applied element:

var myImport = document.head.querySelector('link[rel=import]').import;
var link = myImport.querySelector('link');
console.log(link.__appliedElement || link);
var style = myImport.querySelector('style');
console.log(style.__appliedElement || style);

Building & Running Tests


$ git clone
$ cd html-imports
$ npm i
$ bower i
$ gulp

Run tests

$ npm i -g web-component-tester
$ wct