It is still possible to use login.html, though it is not recommended. The drawback of using a separate login page is that the entire page has to be loaded again after login, and running sessions are not automatically restored (see Michel's comment).
To be able to customize your login form, Michel's solution is the way to go. However, if you really need a separate login page, you can still login using login.html. You will also be returned to that page when logging out.
If you always want to login using the separate login page, but don't want to navigate to login.html by hand, just rename login.html to index.html, and index.html to index3.html. That way you will always start with your customized login page.
You can create an anonymous accessible form, and add a loginform widget to it to achieve a customized login form.