The mediawiki puppet module configures HHVM to use /srv/mediawiki/errorpages/404.php for its error_document404 setting.
Per https://github.com/wikimedia/puppet/blob/373c3e8e5b/modules/mediawiki/manifests/hhvm.pp#L40.
However, when requesting a url that (I believe) is routed by Apache to HHVM, this error template isn't used.
For a url like https://en.wikipedia.org/w/foo.php, some basic default is being served instead:
In order to be able to update the look of this 404 (parent task) we first need to figure out where this response is coming from, and how to change it. If it isn't coming from HHVM, then from where? If it comes from Apache (next best guess), then why isn't Apache using the 404 handler we've configured there?