Sometime in the last week, the handling of the action attribute in a form hasn't been taking into account a <base> tag on the page. With the forms at http://demo.silverstripe.com/Security/login, they should be submitting to http://demo.silverstripe.com/Security/LoginForm, as the base href is http://demo.silverstripe.com/, however they're submitting to http://demo.silverstripe.com/Security/Security/LoginForm, thus rendering the form unusable.
Forgot to mention that this is on a trunk build of WebKit, revision 45757
This bug is a duplicate of bug #27189.
*** This bug has been marked as a duplicate of bug 27189 ***