src/vs/workbench/contrib/webview/browser/webviewElement.ts/WebviewElement/createElement
class WebviewElement:	private createElement(options: WebviewOptions, _contentOptions: WebviewContentOptions) {
		// Do not start loading the webview yet.
		// Wait the end of the ctor when all listeners have been hooked up.
		const element = document.createElement('iframe');
		element.name = this.id;
		element.className = `webview ${options.customClasses || ''}`;
		element.sandbox.add('allow-scripts', 'allow-same-origin', 'allow-forms', 'allow-pointer-lock', 'allow-downloads');
		if (!isFirefox) {
			element.setAttribute('allow', 'clipboard-read; clipboard-write;');
		}
		element.style.border = 'none';
		element.style.width = '100%';
		element.style.height = '100%';

		element.focus = () => {
			this.doFocus();
		};

		return element;
	}