ui.html

<div id='container' tabindex='-1'></div>

<script>
	const container = document.getElementById('container')

	function renderContainer() {
		container.innerText = document.activeElement === container
			? 'you can use keyboard controls'
			: 'focus the plugin to use controls'
	}

	container.addEventListener('focus', renderContainer)
	container.addEventListener('blur', renderContainer)
	renderContainer()

	const pressedKeys = new Set()

	function updatePressedKeys() {
		parent.postMessage({ pluginMessage: { kind: 'UPDATE_PRESSED_KEYS', pressedKeys: [ ...pressedKeys ] } }, '*')
	}

	container.addEventListener('keydown', (event) => {
		pressedKeys.add(event.key)
		updatePressedKeys()
	})

	container.addEventListener('keyup', (event) => {
		pressedKeys.delete(event.key)
		updatePressedKeys()
	})
</script>

<style>
	html, body {
		margin: 0;
		height: 100%;
	}

	#container {
		border: 5px solid red;
		color: red;
		font-family: 'Comic Sans MS', sans-serif;
		text-align: center;
		height: 100%;
		box-sizing: border-box;
		outline: none;
		display: flex;
		flex-direction: column;
		align-items: center;
		justify-content: center;
		opacity: 0.5;
		cursor: default;
		user-select: none;
	}

	#container:focus {
		border-color: green;
		border-width: 15px;
		color: green;
		opacity: 1;
	}
</style>