Have you tried the HtmlAgilityPack? It will allow you to parse and query (with XPATH) a lot of the malformed HTML you find.
If I’m understanding your problem correctly, you might use:
HtmlAgilityPack.HtmlWeb web = new HtmlAgilityPack.HtmlWeb();
HtmlAgilityPack.HtmlDocument doc = web.Load("http://abc.com/xyz.html");
HtmlAgilityPack.HtmlNode div = doc.DocumentNode
.SelectSingleNode("/html/body/div[@class=\"os-box unround\"]");
string contentYouWantedToDisplayOnYourOwnPage = div.InnerHtml;
solved Extracting div’s InnerHtml?