Sometimes one monopoly fades just to make room for another. Just as IE's dominance in the browsing space was no longer being felt, WebKit is here to take its place. Granted, WebKit is not a browser, but it's the engine being used by Chrome, the biggest browser on the web, Safari and many mobile browsers out there.
What this means is that developers optimize their sites for WebKit and everything else is an afterthought. What's worse, developers extensively used -webkit- prefixed properties but don't include the same prefixed properties for other browsers or, worse yet, the unprefixed standard property.
What this led to is Opera starting to use parse -webkit prefixes in cases where the functionality is available in Opera but developers choose not to include the Opera or standard version of the property. Opera 12.50
will be the first desktop release to support -webkit prefixes. More details on what led to the situation and what is being done to fix it are available here