Last modified: 2013-07-25 07:08:43 UTC
$wgHandheldStyle, added by Brion back in 2008 (dc821ec1 / r38116), it slightly useless - it was only ever implemented for Monobook, and the only use-case given was using Chick's CSS for mobile devices with Monobook being the default skin - and now Chick is no more and Monobook isn't even the default.
Apart from that, it's only used to load styles from [[MediaWiki:Handheld.css]] if it's set (ResourceLoaderSiteModule.php).
Either it should be used consistently for all skins, preferably inside ResourceLoader, or it should be killed off. Personally I'd vote for killing.
Related URL: https://gerrit.wikimedia.org/r/60259 (Gerrit Change Ia8d79b4a637a227eb1879fbb874173fdd9bd6d99)
I have tentatively submitted a patch to remove it at Ia8d79b4a, waiting for input.