In Polymer, why use `<input is =" iron-input ">` instead of `<iron-input>`?

Question

In a Polymer document ( https://elements.polymer-project.org/elements/iron-input ) I found:

<input is="iron-input" bind-value="{{myValue}}">

And in another white paper ( https://www.polymer-project.org/1.0/docs/devguide/registering-elements.html#type-extension ) I found:

    <dom-module id="main-document-element">
      <template>
        <p>
          Hi! I'm a Polymer element that was defined in the
          main document!
        </p>
      </template>
      <script>
        HTMLImports.whenReady(function () {
          Polymer({
            is: 'main-document-element'
          });
        });
      </script>
    </dom-module>
    <main-document-element></main-document-element>

I'm just wondering why the first <input is="iron-input" bind-value="{{myValue}}">cannot be written as <iron-input bind-value="{{myValue}}">.

Is it for compatibility, what makes polyfill easier?

Answer 1

The element iron-inputdoes not contain HTML in the source code. This means that execution:

<iron-input bind-value="{{myValue}}">

. iron-input - , HTML.