How to do Forms Authentication on purely HTML pages using ASP.NET?

I am using forms authentication in IIS7 to password-protect a dev site, but the authentication seems to get by-passed when the site contains only static HTML files + login.aspx + web.config.

When I renamed the files to .aspx, I am prompted with the login form I am not doing anything fancy. I have a very simple login script and it should just redirect to index.html afterward.

Any suggestions? To summarize, the entire site is using HTML (for now) and needs to be password protected.

<authentication mode="Forms">
  <forms name="appNameAuth" path="/" loginUrl="~/login.aspx" defaultUrl="index.html" protection="All" timeout="525600">
    <credentials passwordFormat="Clear">
      <user name="[user]" password="[password]" />
    </credentials>
  </forms>
</authentication>
<authorization>
  <deny users="?" />
</authorization>