Just do as follows -
In design page add one literal control -
<body>
<form id="form1" runat="server">
<div>
<asp:Literal ID="htmlbody" runat="server"></asp:Literal>
</div>
</form>
</body>
Then in .cs page -
protected void Page_Load(object sender, EventArgs e)
{
StreamReader sr;
string html;
sr = File.OpenText(Server.MapPath("FileName.html"));
html = sr.ReadToEnd();
sr.Close();
Regex start = new Regex(@"[\s\S]*<body[^<]*>",RegexOptions.IgnoreCase);
html = start.Replace(html, "");
Regex end = new Regex(@"</body[\s\S]*", RegexOptions.IgnoreCase);
html = end.Replace(html, "");
htmlbody.Text = html;
}
It will add the html file contents to the aspx page.