extensions/emmet/src/util.ts/validate
export function validate(allowStylesheet: boolean = true): boolean {
	const editor = vscode.window.activeTextEditor;
	if (!editor) {
		vscode.window.showInformationMessage('No editor is active');
		return false;
	}
	if (!allowStylesheet && isStyleSheet(editor.document.languageId)) {
		return false;
	}
	return true;
}