async embedAdditionalStylesheet(document) {
		const styleSheetsIncluded = [];
		(this.options.additionalStylesheets || []).forEach(cssFile => {
			if (styleSheetsIncluded.includes(cssFile)) {
				return;
			}
			styleSheetsIncluded.push(cssFile);
			const webpackCssAssets = Object.keys(this.compilation.assets).filter(
				file => minimatch(file, cssFile)
			);
			webpackCssAssets.map(asset => {
				const style = document.createElement('style');
				style.$$external = true;
				style.textContent = this.compilation.assets[asset].source();
				document.head.appendChild(style);
			});
		});
	}
Inline the stylesheets from options.additionalStylesheets (assuming it passes `options.filter`)