Revert "Un-taint the base tag with "./" things :)"

This reverts commit 0bc5a56ab751f5a6c98f1ebca4df8964207e99ae.
Revert "Remove the filename from BASE_HREF (used for <base />), fixes bug#66138"

This reverts commit cc3a7dccac0b1723b2778414aea89cde1c62e506.