I think "test.html" is not a valid name for the resource. Instead, try using "test_html". Then the following works very well.
private void button1_Click(object sender, EventArgs e)
{
string html = Properties.Resources.test_html;
webBrowser1.DocumentText = html;
}
So if the HTML file
<!DOCTYPE html>
<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta charset="utf-8" />
<title></title>
</head>
<body>
This is some resource HTML
</body>
</html>
