private async Task<CssUsageInfo[]> AnalyzeExternalStyles(Uri pageUrl, HtmlDocument html)
{
var cssUrls = html
.GetExternalCssUrls()
.Select(href => UrlHelper.CreateFromHref(pageUrl, href))
.ToList();
var tasks = cssUrls.Select(cssUrl => Task.Run(async () =>
{
var stylesheet = await _stylesheets.GetOrDownload(cssUrl);
return AnalyzeCssStylesheet(stylesheet, html);
})).ToList();
return await Task.WhenAll(tasks);
}