$wgHtml5 is usable in 1.17 -- get rid of $wgHtml5 = false in the config files. It was only there because the old 1.16 snapshot used by Wikimedia didn't produce well-formed XML. Even the 1.16 release works fine.
The only possible negative side-effect would be that some pages might not be well-formed XML due to bugs, e.g., if there are named entities that have crept in since r68803. These should be fixable easily on a case-by-case basis. $wgHtml5 = true is the default since 1.16, so we want to test it on the main site and fix any resulting bugs even if there are any.