Manual:$wgCleanupPresentationalAttributes/en

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 wikis 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 cellpadding which aren't easily converted into inline CSS.

This variable only takes effect when is enabled.

Removal
This feature was removed from MediaWiki core because it made a number of incorrect conversions, including at least one that should not have been done at all. Its removal has been backported to MediaWiki 1.20 as part of the MediaWiki 1.20.1 maintenance release.

MediaWiki 1.19 users should disable this feature (by adding  to the end of LocalSettings.php) if they experience problems with the following HTML attributes: "align", "clear", "height", "nowrap", "size", "type", "valign", "width"