You can do it the simple way:
string pathToHTMLFile = @"C:\temp\someFile.html";
string htmlString = File.ReadAllText(pathToHTMLFile);
Or you could stream it in with FileStream/StreamReader:
using (FileStream fs = File.Open(pathToHTMLFile, FileMode.Open, FileAccess.ReadWrite))
{
using (StreamReader sr = new StreamReader(fs))
{
htmlString = sr.ReadToEnd();
}
}
This latter method allows you to open the file while still permitting others to perform Read/Write operations on the file. I can't imagine an HTML file being very big, but it has the added benefit of streaming the file instead of capturing it as one large chunk like the first method.