I think that content type should be explicitly set in login.jsp or in class org.exoplatform.web.login.InitiateLoginServlet. Right now, almost all GateIn pages have correctly set HTTP header Content-Type in HTTP response. But this is not the case for page with login form. Header Content-Type is not set in HTTP response from this page.
This is not the issue if you are sending HTTP requests via your browser directly to GateIn portal, but can be the issue in some special cases. For example if Apache HTTPD is used as loadbalancer and cluster with GateIn portal servers is behind it. Apache HTTPD attaches header "Content-Type" in case that this header is not set from backend application. And by default it set the header to text/plain. Result is that I can see only HTML code in my browser in that case (see screenshot).