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');

		const allowRules = ['cross-origin-isolated', 'autoplay'];
		if (!isFirefox) {
			allowRules.push('clipboard-read', 'clipboard-write');
		}
		element.setAttribute('allow', allowRules.join('; '));

		element.style.border = 'none';
		element.style.width = '100%';
		element.style.height = '100%';

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

		return element;
	}