Few pull requests were merged in prosilver in 3.1.5-RC1 that change code in overall_header.html in head section: https://tracker.phpbb.com/browse/PHPBB3-13873 and https://tracker.phpbb.com/browse/PHPBB3-13876

In both tickets fixes were incomplete: changes were not applied to simple_header.html

Additionally, link code was changed from <link /> to <link>, but it was changed only for few elements, which makes code inconsistent. Since <link> is better suited for html doctype, the rest of entries should be changed as well.