Walk up to the library and dive into some dusty books about the wild early days of CSS.
Also, enjoy the conversation about that line using git blame:
https://we.phorge.it/source/phorge/browse/master/webroot/rsrc/css/aphront/typeahead.css;7aee92b5e95cfc11059e9ff8788e5e53dc88e84d$7
https://we.phorge.it/source/phorge/browse/master/webroot/rsrc/css/aphront/typeahead.css;3cf6f746f0555b4d73d587972d896de99667fb06$7
https://we.phorge.it/source/phorge/browse/master/webroot/rsrc/css/aphront/typeahead.css;7a9d5f8f2de5e44f4014116a47de4ea870361732$7
https://we.phorge.it/source/phorge/browse/master/webroot/rsrc/css/aphront/typeahead.css;0aa67025f25d30162309d59b4cbe133b6180b2d7$7