Manual:$wgCleanupPresentationalAttributes

Details
Converts certain presentational attributes that have been deprecated since HTML5 and were removed in HTML5 (such as valign) into their respective inline CSS styles instead. This allows wiki to keep using them in short loose WikiText but still output valid modern markup. Note that there were some attributes excluded; namely stuff on img and object,, and the table cellspacing and cellpading which aren't easily converted into inline css.

This variable only takes effect when $wgHtml5 is enabled.