Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
-webkit-transition is a non-standard boolean CSS media feature whose value indicates whether vendor-prefixed CSS transitions are supported or not. This media feature is only supported by WebKit. The standards-based alternative is to use a @supports feature query instead.
Specification
Not part of any specification. Apple has a description in Safari CSS Reference.