public interface IHtmlDownloader
{
    IWebProxy Proxy { get; set; }
    string UserAgent { get; set; }

    string GetHtml(string url)
}

public class HtmlDownloader : IHtmlDownloader
{
    WebClient _client;

    public IWebProxy Proxy
    {
        get { return _client.Proxy; }
        set { _client.Proxy = value; }
    }

    public string UserAgent
    {
        get
        { return _client.Headers[HttpRequestHeader.UserAgent].ToString(); }
        set
        { _client.Headers[HttpRequestHeader.UserAgent] = value; }
    }

    public HtmlDownloder()
    {
        _client = new WebClient();
    }

    public string GetHtml(string url)
    {
        this.CheckValidity(url);

        var htmlOfWebsite = _client.DownloadString(url);

        return htmlOfWebsite;
    }
}

{ url = { required = true
        , dataType = ValidationTypes::URL }
}