easierbycode/index.html

## index.html
<!-- based on https://twitter.com/ebidel/status/933496242272747520 -->
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title><responsive-element></title>
<link rel="stylesheet" href="styles.css">
</head>
<body>

<responsive-container id="container1">
  <div>move the &lt;-- frame around or click-me</div>
</responsive-container>

<responsive-container small="400px" id="container2">
  <ul>
    <li>one</li>
    <li>two</li>
    <li>three</li>
    <li>four</li>
    <li>five</li>
    <li>six</li>
    <li>seven</li>
    <li>eight</li>
    <li>nine</li>
    <li>ten</li>
  </ul>
</responsive-container>

<div id="container3">
  <div>right nav</div>
</div>

<script>
const ResizeObservableElement = (superclass) => class extends superclass {
  static get observer() {
    if (!this._observer) {
      // Set up a single RO for all elements that inherit from this class. This
      // has much better performance than creating a separate RO in every
      // element instance. See https://goo.gl/5uLKZN.
      this._observer = new ResizeObserver(entries => {
        for (const entry of entries) {
          // Custom event works for both node.onresize and node.addEventListener('resize') cases.
          const evt = new CustomEvent('resize', {detail: entry, bubbles: false})
          entry.target.dispatchEvent(evt);
        }
      });
    }
    return this._observer;
  }

  constructor() {
    super();
    this.constructor.observer.observe(this);
  }
};

class ResponsiveContainer extends ResizeObservableElement(HTMLElement) {
  static get is() { return 'responsive-container'; }
  get mode() { return this.getAttribute('mode'); }
  set mode(val) {
    val ? this.setAttribute('mode', val) : this.removeAttribute('mode');
  }
  constructor() {
    super();
    this.smallSize = parseInt(this.getAttribute('small')) || 400;
    // Component responds to it's own resizes.
    this.addEventListener('resize', e => {
      const w = e.detail.contentRect.width;
      this.mode = w <= this.smallSize ? 'small' : 'large';
    });
  }
}

customElements.define(ResponsiveContainer.is, ResponsiveContainer);


// Main page can subscribe to component's resize updates too.
document.querySelector('#container2').addEventListener('resize', e => {
  console.log(e.detail.contentRect.width, e.detail.contentRect.height);
});


// Just a tester to see how component responds.
const c = document.querySelector('#container1');
c.addEventListener('click', e => {
  c.classList.add('off');
  setTimeout(() => c.classList.remove('off'), 2000);
});
</script>
</body>
</html>

## styles.css
* {
  box-sizing: border-box;
}
html, body {
  height: 100vh;
  margin: 0;
}
body {
  display: flex;
  font-family: sans-serif;
  color: #455A64;
  background: #fff;
}
ul {
  list-style: none;
  padding: 0;
  width: 100%;
}
li {
  background: #ffff;
  padding: 16px;
  margin: 8px;
}
#container1, #container2, #container3 {
  display: flex;
  align-items: center;
  justify-content: center;
}
reponsive-container {
  display: block;
}
#container1 {
  display: flex;
  height: 100vh;
  width: 300px;
  background: #eee;
  will-change: width;
  transition: width 600ms ease-in-out;
}
#container1.off {
  width: 0;
}
#container2 {
  background: #ffcc00;
  position: relative;
  flex: 1;
  overflow: auto;
  min-width: 100px;
}
#container2::before {
  font-weight: bold;
  text-transform: uppercase;
  background: blue;
  padding: 8px;
  position: absolute;
  content: attr(mode);
  top: 0;
  right: 0;
  transform: rotateZ(45deg) translate(65px, -35px);
  width: 200px;
  text-align: center;
  color: white;
}
#container2[mode="large"] ul {
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
}
#container2[mode="large"] li {
  width: 200px;
  height: 200px;
}
#container3 {
  background: #ccc;
  min-width: 100px;
}
	<!-- based on https://twitter.com/ebidel/status/933496242272747520 -->
	<!DOCTYPE html>
	<html lang="en">
	<head>
	<meta charset="UTF-8">
	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<meta http-equiv="X-UA-Compatible" content="ie=edge">
	<title><responsive-element></title>
	<link rel="stylesheet" href="styles.css">
	</head>
	<body>

	<responsive-container id="container1">
	<div>move the <-- frame around or click-me</div>
	</responsive-container>

	<responsive-container small="400px" id="container2">
	<ul>
	<li>one</li>
	<li>two</li>
	<li>three</li>
	<li>four</li>
	<li>five</li>
	<li>six</li>
	<li>seven</li>
	<li>eight</li>
	<li>nine</li>
	<li>ten</li>
	</ul>
	</responsive-container>

	<div id="container3">
	<div>right nav</div>
	</div>

	<script>
	const ResizeObservableElement = (superclass) => class extends superclass {
	static get observer() {
	if (!this._observer) {
	// Set up a single RO for all elements that inherit from this class. This
	// has much better performance than creating a separate RO in every
	// element instance. See https://goo.gl/5uLKZN.
	this._observer = new ResizeObserver(entries => {
	for (const entry of entries) {
	// Custom event works for both node.onresize and node.addEventListener('resize') cases.
	const evt = new CustomEvent('resize', {detail: entry, bubbles: false})
	entry.target.dispatchEvent(evt);
	}
	});
	}
	return this._observer;
	}

	constructor() {
	super();
	this.constructor.observer.observe(this);
	}
	};

	class ResponsiveContainer extends ResizeObservableElement(HTMLElement) {
	static get is() { return 'responsive-container'; }
	get mode() { return this.getAttribute('mode'); }
	set mode(val) {
	val ? this.setAttribute('mode', val) : this.removeAttribute('mode');
	}
	constructor() {
	super();
	this.smallSize = parseInt(this.getAttribute('small')) \|\| 400;
	// Component responds to it's own resizes.
	this.addEventListener('resize', e => {
	const w = e.detail.contentRect.width;
	this.mode = w <= this.smallSize ? 'small' : 'large';
	});
	}
	}

	customElements.define(ResponsiveContainer.is, ResponsiveContainer);


	// Main page can subscribe to component's resize updates too.
	document.querySelector('#container2').addEventListener('resize', e => {
	console.log(e.detail.contentRect.width, e.detail.contentRect.height);
	});


	// Just a tester to see how component responds.
	const c = document.querySelector('#container1');
	c.addEventListener('click', e => {
	c.classList.add('off');
	setTimeout(() => c.classList.remove('off'), 2000);
	});
	</script>
	</body>
	</html>
	* {
	box-sizing: border-box;
	}
	html, body {
	height: 100vh;
	margin: 0;
	}
	body {
	display: flex;
	font-family: sans-serif;
	color: #455A64;
	background: #fff;
	}
	ul {
	list-style: none;
	padding: 0;
	width: 100%;
	}
	li {
	background: #ffff;
	padding: 16px;
	margin: 8px;
	}
	#container1, #container2, #container3 {
	display: flex;
	align-items: center;
	justify-content: center;
	}
	reponsive-container {
	display: block;
	}
	#container1 {
	display: flex;
	height: 100vh;
	width: 300px;
	background: #eee;
	will-change: width;
	transition: width 600ms ease-in-out;
	}
	#container1.off {
	width: 0;
	}
	#container2 {
	background: #ffcc00;
	position: relative;
	flex: 1;
	overflow: auto;
	min-width: 100px;
	}
	#container2::before {
	font-weight: bold;
	text-transform: uppercase;
	background: blue;
	padding: 8px;
	position: absolute;
	content: attr(mode);
	top: 0;
	right: 0;
	transform: rotateZ(45deg) translate(65px, -35px);
	width: 200px;
	text-align: center;
	color: white;
	}
	#container2[mode="large"] ul {
	display: flex;
	justify-content: center;
	flex-wrap: wrap;
	}
	#container2[mode="large"] li {
	width: 200px;
	height: 200px;
	}
	#container3 {
	background: #ccc;
	min-width: 100px;
	}