src/vs/workbench/contrib/webview/browser/pre/index.html

<!DOCTYPE html>
<html lang="en" style="width: 100%; height: 100%;">

<head>
	<meta charset="UTF-8">
	<meta http-equiv="Content-Security-Policy"
		content="default-src *; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'" />

	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<meta http-equiv="X-UA-Compatible" content="ie=edge">
	<title>Virtual Document</title>
</head>

<body style="margin: 0; overflow: hidden; width: 100%; height: 100%">
	<script src="main.js"></script>
	<script>
		(function () {
			const id = document.location.search.match(/\bid=([\w\-]+)/)[1];
			const handlers = new Map();

			const postMessageToVsCode = (channel, data) => {
				window.parent.postMessage({ target: id, channel, data }, '*');
			};

			window.addEventListener('message', (e) => {
				if (e.data && (e.data.command === 'onmessage' || e.data.command === 'do-update-state')) {
					// Came from inner iframe
					postMessageToVsCode(e.data.command, e.data.data);
					return;
				}

				const channel = e.data.channel;
				const handler = handlers.get(channel);
				if (handler) {
					handler(e, e.data.args);
				} else {
					console.log('no handler for ', e);
				}
			});

			createWebviewManager({
				origin: document.location.href,
				postMessage: (channel, data) => {
					postMessageToVsCode(channel, data);
				},
				onMessage: (channel, handler) => {
					handlers.set(channel, handler);
				}
			});
		}());
	</script>
</body>
</html>