For web-compatibility reasons, Blink, WebKit, and Gecko browsers treat all pseudo-elements starting with ::-webkit- as valid. If there is an invalid pseudo-element or pseudo-class within in a chain or group of selectors, the whole selector list is invalid
These are the vendor-prefixed properties offered by the relevant rendering engines (-webkit for Chrome, Safari; -moz for Firefox, -o for Opera, -ms for Internet Explorer). Typically they're used to implement new, or proprietary CSS features, prior to