polymer

Build modern apps using web components


Keywords
polymer, web-components
License
Other
Install
bower install polymer

Contributors

Steve Orvell Kevin Schaaf Daniel Freedman Scott J. Miles Frankie Fu Josh Soref Eric Bidelman Peter Burns Tim van der Lippe Herr Kaste Nazar Mokrynskyi Arthur Evans Taylor Savage Monica Dinculescu Emmanuel Garcia Russell Bicknell Rafael Weinstein Ian MacLeod Valdrin Koshi Addy Osmani Justin Fagnani Elliott Marquez Jennifer Messerly Rob Dodson


See all contributors


Documentation

Polymer

Build Status

Polymer lets you build encapsulated, reusable elements that work just like standard HTML elements, to use in building web applications.

<!-- Polyfill Web Components for older browsers -->
<script src="webcomponentsjs/webcomponents-lite.js"></script>

<!-- Import element -->
<link rel="import" href="google-map.html">

<!-- Use element -->
<google-map latitude="37.790" longitude="-122.390"></google-map>

Check out polymer-project.org for all of the library documentation, including getting started guides, tutorials, developer reference, and more.

Or if you'd just like to download the library, check out our releases page.

Polymer 2.0 is out!

Polymer 2.0 is released, and will be the future focus of Polymer development going forward. We intend to keep the 2.x public API stable barring critical issues. For background and migration information on the 2.x see the 2.0 documentation on the website or the 2.0 section below, and we welcome your feedback via issues or Slack.

To evaluate Polymer 2.0, please point your bower at the latest 2.0.0 tag for polymer, and be sure load to the webcomponentsjs/webcomponents-lite.js or webcomponentsjs/webcomponents-loader.js polyfills from the latest v1.0.0 tag of webcomponentsjs.

👀 Looking for Polymer v1.x? Please see the the v1 branch.

Overview

Polymer is a lightweight library built on top of the web standards-based Web Components APIs, and makes it easier to build your very own custom HTML elements. Creating reusable custom elements - and using elements built by others - can make building complex web applications easier and more efficient. By being based on the Web Components API's built in the browser (or polyfilled where needed), Polymer elements are interoperable at the browser level, and can be used with other frameworks or libraries that work with modern browsers.

Among many ways to leverage custom elements, they can be particularly useful for building reusable UI components. Instead of continually re-building a specific navigation bar or button in different frameworks and for different projects, you can define this element once using Polymer, and then reuse it throughout your project or in any future project.

Polymer provides a declarative syntax to easily create your own custom elements, using all standard web technologies - define the structure of the element with HTML, style it with CSS, and add interactions to the element with JavaScript.

Polymer also provides optional two-way data-binding, meaning:

  1. When properties in the model for an element get updated, the element can update itself in response.
  2. When the element is updated internally, the changes can be propagated back to the model.

Polymer is designed to be flexible, lightweight, and close to the web platform - the library doesn't invent complex new abstractions and magic, but uses the best features of the web platform in straightforward ways to simply sugar the creation of custom elements.

In addition to the Polymer library for building your own custom elements, the Polymer project includes a collection of pre-built elements that you can drop on a page and use immediately, or use as starting points for your own custom elements.

Polymer in 1 Minute

Polymer adds convenient features to make it easy to build complex elements:

Basic custom element without Polymer:

// Standard custom element that Extends HTMLElement
class MyElement extends HTMLElement {
  constructor() {
    super();
    console.log('my-element was created!');
  }
}

// Register custom element class with browser
customElements.define('my-element', MyElement);
<!-- use the element -->
<my-element></my-element>

Custom element using Polymer

<!-- Define template that your element will use -->
<dom-module id="my-simple-namecard">
  <template>
    <div>
      Hi! My name is <span>Jane</span>
    </div>
  </template>
</dom-module>
// Custom element that extends Polymer base class
class MySimpleNamecard extends Polymer.Element {

  // Stamp template from this dom-module into element's shadow DOM:
  static get is() { return 'my-simple-namecard'; }

}

// Register custom element class with browser
customElements.define(MySimpleNamecard.is, MySimpleNamecard);

Configure properties on your element...

// Create an element that takes a property
class MyPropertyNamecard extends Polymer.Element {

  static get is() { return 'my-property-namecard'; }

  // Define property/attribute API:
  static get properties() {
    return {
      myName: {
        type: String,
        observer: 'myNameChanged'
      }
    };
  }

  myNameChanged(myName) {
    this.textContent = 'Hi! My name is ' + myName;
  }

}

customElements.define(MyPropertyNamecard.is, MyPropertyNamecard);

...and have them set using declarative attributes

<!-- using the element -->
<my-property-namecard my-name="Jim"></my-property-namecard>

Hi! My name is Jim

Bind data into your element using the familiar mustache-syntax

<!-- Define template with bindings -->
<dom-module id="my-bound-namecard">
  <template>
    <div>
      Hi! My name is <span>[[myName]]</span>
    </div>
  </template>
</dom-module>
class MyBoundNamecard extends Polymer.Element {

  static get is() { return 'my-bound-namecard'; }

  static get properties() {
    return {
      myName: String
    };
  }

}

customElements.define(MyBoundNamecard.is, MyBoundNamecard);
<!-- using the element -->
<my-bound-namecard my-name="Josh"></my-bound-namecard>

Hi! My name is Josh

Style the internals of your element, without the style leaking out

<!-- Add style to your element -->
<dom-module id="my-styled-namecard">
  <template>
    <style>
      /* This would be crazy without webcomponents, but with shadow DOM */
      /* it only applies to this element's private "shadow DOM" */
      span {
        font-weight: bold;
      }
    </style>

    <div>
      Hi! My name is <span>{{myName}}</span>
    </div>
  </template>
</dom-module>
class MyStyledNamecard extends Polymer.Element {

  static get is() { return 'my-styled-namecard'; }

  static get properties() {
    return {
      myName: String
    };
  }

}

customElements.define(MyStyledNamecard.is, MyStyledNamecard);
<!-- using the element -->
<my-styled-namecard my-name="Jesse"></my-styled-namecard>

Hi! My name is Jesse

and so much more!

Web components are an incredibly powerful new set of primitives baked into the web platform, and open up a whole new world of possibility when it comes to componentizing front-end code and easily creating powerful, immersive, app-like experiences on the web.

By being based on Web Components, elements built with Polymer are:

  • Built from the platform up
  • Self-contained
  • Don't require an overarching framework - are interoperable across frameworks
  • Re-usable

Contributing

The Polymer team loves contributions from the community! Take a look at our contributing guide for more information on how to contribute.

Communicating with the Polymer team

Beyond GitHub, we try to have a variety of different lines of communication available:

License

The Polymer library uses a BSD-like license that is available here


Polymer 2.0

Polymer 2.0 is a major new release of Polymer that is compatible with the latest web components standards and web platform APIs, and makes significant improvements over the 1.x version of the library. The following section provides context and migration information for existing users of Polymer 1.x:

Goals of Polymer 2.0

  1. Take advantage of native "v1" Web Components implementations across browsers.

    The primary goal of the Polymer 2.0 release is to take advantage of native, cross-browser support for Web Components.

    Polymer 1.x is built on top of the so-called "v0" Web Components specs, which are supported natively only in Google Chrome; using Polymer in other browsers has always required the use of polyfills.

    Beginning this fall, multiple browsers will be shipping native implementations of the new "v1" specs for Shadow DOM and Custom Elements, yielding better web components performance and reducing the need for polyfills.

    Polymer 2.0 features full support for the v1 specs, taking advantage of native browser implementations where they are available and depending on updated v1 polyfills from webcomponentsjs where necessary.

    Polymer 2.0 also embraces the new ES-class-based mechanism for defining custom elements, bringing idiomatic Polymer style closer to "vanilla" custom element authoring.

  2. Provide a smooth migration path from Polymer 1.x.

    Our second major goal is to provide as easy a transition as possible for developers who have built elements and apps with Polymer 1.x, making Polymer 2.0 a sturdy bridge to the future.

    To upgrade, you will need to make some changes to your 1.x-based elements and apps. These changes are necessitated by both the v0-to-v1 spec transition and a handful of key improvements in Polymer itself (see our remaining goals, below).

    However, we've taken care to limit the number of changes that are strictly required and to ease the process of upgrading:

    • Polymer 2.0 introduces a new ES6 class-based syntax, but we've provided a lightweight compatibility layer allowing you to upgrade your 1.x code with minimal modifications. Depending on your needs, you can either take advantage of the compatibility layer or jump straight to idiomatic 2.0 style.

    • Before releasing Polymer 2.0, we'll also provide an upgrade tool to automate as many of the changes (both required and recommended) as possible.

    • Finally, we're working on guidelines for building and testing "hybrid" elements that will run in both Polymer 1.x and Polymer 2.0. We plan to ship hybrid versions of all of the elements that we provide, easing the transition for developers who use them. Third-party element providers may also choose to ship hybrid elements.

    • If you have an especially large app or constraints that don't allow for an all-at-once upgrade, you can also use hybrid elements to migrate your app from 1.x to 2.0 in piecewise fashion: update your elements to hybrid form, individually or in batches, while running against Polymer 1.x; then cut over to Polymer 2.0 when all of your elements have been updated.

  3. Eliminate leaky abstractions.

    Seamless interoperability is one of Web Components' major selling points. Generally speaking, web components "just work" anywhere you use HTML elements. To use them, you need only be aware of their public attributes, properties, methods and events; you don't need to know anything about their inner workings. This means you can easily mix standard HTML elements, third-party elements and elements you've defined yourself.

    Unfortunately, there are a couple of cases in Polymer 1.x (the Polymer.dom API and the set/notifyPath API) where implementation details of Polymer-based elements leak out, requiring users of the elements to interact with them in non-standard ways. These "leaks" were by design – compromises we chose to make in the interest of performance – but in hindsight we aren't happy with the tradeoff.

    In Polymer 2.0 we've found ways to eliminate these leaky abstractions without unduly compromising performance, which means that your Polymer 2.x-based elements will be indistinguishable from "vanilla" elements from a consumer's point of view (unless you leak implementation details of your own).

  4. Make targeted improvements to the Polymer data system.

    Based on developer feedback and observations of Polymer apps in the wild, we've also made some key improvements to Polymer's data system. These changes are designed to make it easier to reason about and debug the propagation of data through and between elements:

    • Changes are now batched, and the effects of those changes are run in well-defined order.
    • We ensure that multi-property observers run exactly once per turn for any set of changes to dependencies (removing the multi-property undefined rule).
    • To add compatibility with more approaches to state management, we now provide a mixin (and legacy behavior) to skip dirty-checking properties whose values are objects or arrays and always consider them dirty, causing their side effects to run.
  5. Improve factoring of Polymer and the polyfills

    We've done major refactoring of Polymer and the webcomponentsjs polyfills to improve efficiency, utility and flexibility:

    • The "Shady DOM" shim that was part of Polymer 1.x has been factored out of Polymer and added to the webcomponentsjs polyfills, along with the related shim for CSS Custom Properties. (As noted above, the Shady DOM shim no longer exposes an alternative API but instead patches the native DOM API for transparent usage).

    • Polymer itself has been internally factored into several loosely coupled libraries.

      • The new Polymer.Element class extends from the native HTMLElement and mixes in functionality from these libraries.

      • The idiomatic way of using Polymer 2.0 (assuming you're not using the 1.x compatibility layer) is to define your own custom elements that subclass Polymer.Element, using standard ES class definition syntax.

      • If you're interested in using pieces of Polymer's functionality in a la carte fashion, you can try defining your own base element class, utilizing a subset of the libraries. For now, this use case should be considered experimental, as the factoring of libraries is subject to change and is not part of the official Polymer 2.0 API.

1.0 Compatibility Layer

Polymer 2.0 retains the existing polymer/polymer.html import that current Polymer 1.0 users can continue to import, which strives to provide a very minimally-breaking change for code written to the Polymer 1.0 API. For the most part, existing users upgrading to Polymer 2.0 will only need to adapt existing code to be compliant with the V1 Shadow DOM API related to content distribution and styling, as well as minor breaking changes introduced due to changes in the V1 Custom Elements spec and data-layer improvements listed below.

2.0 ES6 Class-based Syntax

With the widespread adoption of ES6 in browsers, as well as the requirement that V1 Custom Elements be defined as ES6 class extensions of HTMLElement, Polymer 2.0 shifts its primary API for defining new elements to an ES6 class-centric syntax. Using this syntax, users will extend Polymer.Element (a subclass of HTMLElement), which provides meta-programming for most of the same features of Polymer 1.0 based on static configuration data supplied on the class definition.

Basic syntax looks like this:

<!-- Load the Polymer.Element base class -->
<link rel="import" href="bower_components/polymer/polymer-element.html">
// Extend Polymer.Element base class
class MyElement extends Polymer.Element {
  static get is() { return 'my-element'; }
  static get properties() { return { /* properties metadata */ } }
  static get observers() { return [ /* observer descriptors */ ] }
  constructor() {
    super();
    ...
  }
  connectedCallback() {
    super.connectedCallback();
    ...
  }
  ...
}

// Register custom element definition using standard platform API
customElements.define(MyElement.is, MyElement);

Users can then leverage native subclassing support provided by ES6 to extend and customize existing elements defined using ES6 syntax:

// Subclass existing element
class MyElementSubclass extends MyElement {
  static get is() { return 'my-element-subclass'; }
  static get properties() { return { /* properties metadata */ } }
  static get observers() { return [ /* observer descriptors */ ] }
  constructor() {
    super();
    ...
  }
  ...
}

// Register custom element definition using standard platform API
customElements.define(MyElementSubclass.is, MyElementSubclass);

Below are the general steps for defining a custom element using this new syntax:

  • Extend from Polymer.Element. This class provides the minimal surface area to integrate with 2.0's data binding system. It provides only standard custom element lifecycle with the addition of ready. (You can apply the Polymer.LegacyElementMixin to get all of the Polymer 1.0 element API, but since most of this API was rarely used, this should not often be needed.)
  • Implement "behaviors" as mixins that return class expressions and apply to the base class you are extending from.
  • Property metadata (properties) and multi-property/wildcard observers (observers) should be put on the class as static getters, but otherwise match the 1.x syntax.
  • In order to provide a template to stamp into the element's shadow DOM, either define a static is getter that returns the id of a dom-module containing the element's template, or else provide a static template getter that returns a template to stamp. The template getter may either return an HTMLTemplateElement or a string containing HTML which will be parsed into a template.
  • listeners and hostAttributes have been removed from element metadata; listeners and attributes should be installed using standard platform API (this.addEventListener, this.setAttribute) how and when needed (e.g. in connectedCallback). For convenience _ensureAttribute is available that sets an attribute if and only if the element does not yet have that attribute, to match hostAttribute semantics.

Note that Polymer.Element provides a cleaner base class void of a lot of sugared utility API that present on elements defined with Polymer(), such as fire, transform, etc. With web platform surface area becoming far more stable across browsers, we intend to hew towards sugaring less and embracing the raw platform API more. So when using Polymer.Element, instead of using the legacy this.fire('some-event') API, simply use the equivalent platform API's such as this.dispatchEvent(new CustomEvent('some-event', {bubbles: true}). #usetheplatform

See below for a visual guide on migrating Polymer 1.0's declarative syntax to the ES6 class syntax in Polymer 2.0:

Difference in Polymer 1.0 and Polymer 2.0 element definition

Polyfills

Polymer 2.0 has been developed alongside and tested with a new suite of V1-spec compatible polyfills for Custom Elements and Shadow DOM. Polymer 2.0 is compatible the latest releases of webcomponentsjs/webcomponents-lite.js, which is included as a bower dependency to Polymer 2.x.

Breaking Changes

Below is a list of intentional breaking changes made in Polymer 2.0, along with their rationale/justification and migration guidance. If you find changes that broke existing code not documented here, please file an issue and we'll investigate to determine whether they are expected/intentional or not.

Polymer.dom

On browsers that lack native V1 Shadow DOM support, Polymer 2.0 is designed to be used with the new V1 Shady DOM shim, which patches native DOM API as necessary to be mostly equivalent to native Shadow DOM. This removes the requirement to use the Polymer.dom API when interacting with the DOM. Polymer.dom can be eliminated for elements targeting Polymer 2.0, in favor of the native DOM API's.

Note that Polymer.dom is still provided in the polymer.html backward-compatibility layer which simply facades the native API, but usage of it in 2.0 can be removed. Note that Polymer.dom will no longer return Arrays for API's where the platform returns e.g. NodeList's, so code may need to be updated to avoid direct use of array methods.

V1 Shadow DOM

Polymer 2.0 elements will stamp their templates into shadow roots created using V1's attachShadow({mode: 'open'}) by default. As such, user code related to scoped styling, distribution, and events must be adapted to native V1 API. For a great writeup on all Shadow DOM V1 spec changes, see this writeup. Required changes for V1 are summarized below:

Distribution

  • <content> insertion points must be changed to <slot>
  • Insertion points that selected content via <content select="..."> must be changed to named slots: <slot name="...">
  • Selection of distributed content into named slots must use slot="..." rather than tag/class/attributes selected by <content>
  • Re-distributing content by placing a <slot> into an element that itself has named slots requires placing a name attribute on the <slot> to indicate what content it selects from its host children, and placing a slot attribute to indicate where its selected content should be slotted into its parent
  • In the V1 "Shady DOM" shim, initial distribution of children into <slot> is asynchronous (microtask) to creating the shadowRoot, meaning distribution occurs after observers/ready (in Polymer 1.0's shim, initial distribution occurred before ready). In order to force distribution synchronously, call ShadyDOM.flush().
  • Calling Polymer.dom.flush no longer results in callbacks registered with Polymer.dom.observeNodes being called. Instead, the object returned from Polymer.dom.observeNodes now contains a flush method which can be used to immediately call the registered callback if any changes are pending.

Scoped styling

  • ::content CSS pseudo-selectors must be changed to ::slotted, and may only target immediate children and use no descendant selectors
  • :host-context() pseudo-selectors have been removed. These were primarily useful for writing bidi rules (e.g. :host-context([dir=rtl])); these should be replaced with the new :dir(rtl) selector, which we plan to polyfill in the styling shim soon
  • The previously deprecated /deep/ and ::shadow selectors have been completely removed from V1 native support and must not be used (use CSS custom properties to customize styling instead)

Scoped events

  • Code using Polymer.dom(event).localTarget should change to the V1 standard API event.target
  • Code using Polymer.dom(event).path (aka V0 event.path) should change to the V1 standard API event.composedPath()
  • Code using Polymer.dom(event).rootTarget (aka V0 event.path[0]) should change to the V1 standard API event.composedPath()[0]

V1 Custom Elements

Polymer 2.0 elements will target the V1 Custom Elements API, which primarily changes the "created" step to actually invoke the class constructor, imposes new restrictions on what can be done in the constructor (previously createdCallback), and introduces different callback names.

  • Changes to callback names:

    • When using Polymer({...}) from the compatibility layer, all callbacks should use legacy Polymer API names (created, attached, detached, attributeChanged)
    • When extending from Polymer.Element, users should override the V1 standard callback names and call super():
      • created changes to constructor
      • attached changes to connectedCallback
      • detached changes to disconnectedCallback
      • attributeChanged changes to attributeChangedCallback
  • The V1 Custom Elements spec forbids reading attributes, children, or parent information from the DOM API in the constructor (or created when using the legacy API). Likewise, attributes and children may not be added in the constructor. Any such work must be deferred (e.g. until connectedCallback or microtask/setTimeout/requestAnimationFrame).

  • Polymer will no longer produce type-extension elements (aka is="..."). Although they are still included in the V1 Custom Elements spec and scheduled for implementation in Chrome, because Apple has stated it will not implement is, we will not be encouraging its use to avoid indefinite reliance on the Custom Elements polyfill. Instead, a wrapper custom element can surround a native element, e.g. <a is="my-anchor">...</a> could become <my-anchor><a>...</a></my-anchor>. Users will need to change existing is elements where necessary.

  • All template type extensions provided by Polymer have now been changed to standard custom elements that take a <template> in their light dom, e.g.

    <template is="dom-repeat" items="{{items}}">...</template>

    should change to

    <dom-repeat items="{{items}}">
        <template>...</template>
    </dom-repeat>

    For the time being, Polymer (both legacy and class API) will automatically wrap template extensions used in Polymer element templates during template processing for backward-compatibility, although we may decide to remove this auto-wrapping in the future. Templates used in the main document must be manually wrapped.

  • The custom-style element has also been changed to a standard custom element that must wrap a style element e.g.

    <style is="custom-style">...</style>

    should change to

    <custom-style>
      <style>...</style>
    </custom-style>

CSS Custom Property Shim

Polymer 2.0 will continue to use a shim to provide limited CSS Custom Properties support on browsers that do not yet natively support custom properties, to allow an element to expose a custom styling API. The following changes have been made in the shim that Polymer 2.0 will use:

  • The shim will now always use native CSS Custom Properties by default on browsers that implement them (this was opt-in in 1.0). The shim will perform a one-time transformation of stylesheets containing CSS Custom Property mixins to leverage individual native CSS properties where possible for better performance. This introduces some limitations to be aware of.
  • The following invalid styling syntax was previously accepted by the 1.0 custom property shim. In order to support native CSS Custom Properties, rules should be correct to use only natively valid syntax:
    • :root {}
      • Should be :host > * {} (in a shadow root)
      • Should be html {} (in main document)
      • Thus, cannot share old :root styles for use in both main document and shadow root
    • var(--a, --b)
      • Should be var(--a, var(--b))
    • @apply(--foo)
      • Should be @apply --foo;
  • element.customStyle as an object that can be assigned to has been removed; use element.updateStyles({...}) instead.
  • <style> inside of a <dom-module>, but outside of <template> is no longer supported

Data system

Removed API

  • Polymer.instanceof and Polymer.isInstance: no longer needed, use instanceof and instanceof Polymer.Element instead.
  • dom-module: Removed ability to use is and name attribute to configure the module name. The only supported declarative way set the module id is to use id.
  • element.getPropertyInfo: This api returned unexpected information some of the time and was rarely used.
  • element.getNativePrototype: Removed because it is no longer needed for internal code and was unused by users.
  • element.beforeRegister: This was originally added for metadata compatibility with ES6 classes. We now prefer users create ES6 classes by extending Polymer.Element, specifying metadata in the static properties, observers, and is properties. For legacy use via Polymer({...}), dynamic effects may still be added by using beforeRegister but it is now equivalent to the registered lifecycle method. An element's is property cannot be set in beforeRegister as it could in Polymer 1.x.
  • element.attributeFollows: Removed due to disuse.
  • element.classFollows: Removed due to disuse.
  • element.copyOwnProperty: Removed due to disuse.
  • listeners: Removed ability to use id.event to add listeners to elements in local dom. Use declarative template event handlers instead.
  • Methods starting with _ are not guaranteed to exist (most have been removed)

Other

  • Polymer 2.0 uses ES2015 syntax, and can be run without transpilation in current Chrome, Safari 10, Safari Technology Preview, Firefox, and Edge. Transpilation is required to run in IE11 and Safari 9 (as well as Edge for high reliability, due to certain bugs in their ES6 implementation). The polymer-cli tools such as polymer serve and polymer build have built-in support for transpiling when needed.

  • The attached legacy callback is no longer deferred until first render time; it now runs at the time of the native connectedCallback, which may be before styles have resolved and measurement is possible. Instead when measurement is needed use Polymer.RenderStatus.beforeNextRender.

  • The legacy created callback is no longer called before default values in properties have been set. As such, you should not rely on properties set in created from within value functions that define property defaults. However, you can now set any property defaults within the created callback (in 1.0 this was forbidden for observed properties) in lieu of using the value function in properties.

  • Binding a default value of false via an attribute binding to a boolean property will not override a default true property of the target, due to the semantics of boolean attributes. In general, property binding should always be used when possible, and will avoid such situations.

  • lazyRegister option removed and all meta-programming (parsing template, creating accessors on prototype, etc.) is deferred until the first instance of the element is created

  • In Polymer 1.x, the class attribute was explicitly blacklisted from hostAttributes and never serialized. This is no longer the case using the 2.0 legacy API.

  • In Polymer 1.x, URLs in attributes and styles inside element templates were re-written to be relative to the element HTMLImport. Based on user feedback, we are changing this behavior.

    Two new properties are being added to Polymer.Element: importPath and rootPath. The importPath property is a static getter on the element class that defaults to the element HTMLImport document URL and is overridable. It may be useful to override importPath when an element template is not retrieved from a dom-module or the element is not defined using an HTMLImport. The rootPath property is set to the value of Polymer.rootPath which is globally settable and defaults to the main document URL. It may be useful to set Polymer.rootPath to provide a stable application mount path when using client side routing. URL's in styles are re-written to be relative to the importPath property. Inside element templates, URLs in element attributes are no longer re-written. Instead, they should be bound using importPath and rootPath where appropriate. For example:

    A Polymer 1.x template that included:

    <img src="foo.jpg">

    in Polymer 2.x should be:

    <img src$="[[importPath]]foo.jpg">