When the form gets submitted, the form data is now posted to index3.html (action="index3.html"), which does not exist in a default configuration, or is a plain html file, extracted from a theme.
If you want to customize the login form, I suggest to either use a login widget from the App Store, or customize the default login.html. In the latter case, you might want to rename index.html to index3.html and login.html to index.html, so you don't have to navigate to /login.html by hand.
It's fixed. Changed the line with the script to:
< a style="cursor:pointer;" onclick="document.getElementById('username').value='MxAdmin'; document.getElementById('password').value='1'; document.getElementById('submit').click()">Login as MxAdmin
Special thanks to Michel Weststrate :)
This thread can now be closed.