what does -webkit means

Answer 1

WebKit is a HTML/CSS web browser rendering engine for Safari.

Every browser is backed by a rendering engine to draw the HTML/CSS web page.

IE → Trident (discontinued)
Edge → EdgeHTML (clean-up fork of Trident)
Firefox → Gecko
Opera → Presto (no longer use Presto since Feb 2013, consider Opera = Chrome nowadays)
Safari → WebKit
Chrome → Blink (a fork of WebKit).

Answer 2

css 3 basics, 2 lesson