HTML Document Extensions Methods

public static void AddScript(this HtmlDocument htmlDocument, string javaScript)
{
    HtmlElement head = htmlDocument.GetElementsByTagName("head")[0];
    HtmlElement scriptElement = htmlDocument.CreateElement("script");
    IHTMLScriptElement element = (IHTMLScriptElement)scriptElement.DomElement;
    element.text = javaScript;
    head.AppendChild(scriptElement);
}
public static void AddCSS(this HtmlDocument htmlDocument, string cssFileName)
{
    IHTMLDocument2 currentDocument = (IHTMLDocument2)htmlDocument.DomDocument;
    int length = currentDocument.styleSheets.length;
    IHTMLStyleSheet styleSheet = currentDocument.createStyleSheet(@"", length + 1);
    using (TextReader reader = new StreamReader(cssFileName))
    {
        styleSheet.cssText = reader.ReadToEnd();
    }
}
public static void DoPostBack(this HtmlDocument document)
{
    document.InvokeScript("__doPostBack");
}
public static void DoPostBack(this HtmlDocument document, string eventTarget, string eventArgument)
{
    document.InvokeScript("__doPostBack", new object[] { eventTarget, eventArgument });
}
public static HtmlElement GetElementByTitle(this HtmlDocument document, string title)
{
    return (from x in document.All.Cast<HtmlElement>()                    where x.GetAttribute("title") == title                    select x).SingleOrDefault();
}