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

Question

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

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

And in another official document (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 was just wondering why the first one <input is="iron-input" bind-value="{{myValue}}"> can't be written as <iron-input bind-value="{{myValue}}">.

Is it for compatibility, which makes it easier to polyfill?

Answer 1

The iron-input element does not contain any HTML in its source code. This means that doing:

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

will not produce an actual input on the page for the user to interact with. The iron-input element is really a collection of behaviours that you can apply to a standard HTML input.