@polymer/iron-fit-behavior

npm install --save @polymer/iron-fit-behavior

import {PolymerElement} from '@polymer/polymer/polymer-element.js';
import {mixinBehaviors} from '@polymer/polymer/lib/legacy/class.js';
import {html} from '@polymer/polymer/lib/utils/html-tag.js';

import {IronFitBehavior} from '@polymer/iron-fit-behavior/iron-fit-behavior.js';

class SimpleFit extends mixinBehaviors(IronFitBehavior, PolymerElement) {
  static get template() {
    return html`
      <style>
        :host {
          background: lightblue;
          padding: 2px;
        }
      </style>
      verticalAlign: [[verticalAlign]], horizontalAlign: [[horizontalAlign]]
    `;
  }
}

customElements.define('simple-fit', SimpleFit);

<script type="module" src="./simple-fit.js"></script>

<style>
#container {
  margin: 3em;
  border: 2px dashed gray;
  padding: 3em;
}
</style>

<div id="container">
  The <code>&lt;simple-fit&gt;</code> below will be positioned within this div.
  <simple-fit id="simpleFitElement"
    vertical-align="bottom"
    horizontal-align="left"
    auto-fit-on-attach
  ></simple-fit>
</div>

git clone https://github.com/PolymerElements/iron-fit-behavior
cd iron-fit-behavior
npm install
npm install -g polymer-cli

polymer serve --npm
open http://127.0.0.1:<port>/demo/

polymer test --npm