What is WebKit and how is it related to CSS?

Update: So apparently, WebKit is a HTML/CSS web browser rendering engine for Safari/Chrome. Are there such engines for IE/Opera/Firefox and what are the differences, pros and cons of using one over the other? Can I use WebKit features in Firefox for example?

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) (Edge switched to Blink in 2019)
• Firefox → Gecko
• Opera → Presto (no longer uses Presto since Feb 2013, consider Opera = Chrome, therefore Blink nowadays)
• Safari → WebKit
• Chrome → Blink (a fork of Webkit).

See Comparison of web browser engines for a list of comparisons in different areas.

The ultimate question... is WebKit supported by IE?

Not natively.

Addition to what @KennyTM said:

• IE
  • Engine: Trident
  • CSS-prefix: -ms
• Edge
  • Engine: EdgeHTML → Blink³
  • CSS-prefix: -ms
• Firefox
  • Engine: Gecko
  • CSS-prefix: -moz
• Opera
  • Engine: Presto → Blink¹
  • CSS-prefix: -o (Presto) and -webkit (Blink)
• Safari
  • Engine: WebKit
  • CSS-prefix: -webkit
• Chrome
  • Engine: WebKit → Blink²
  • CSS-prefix: -webkit

¹⁾ On February 12 2013 Opera (version 15+) announces they moving away from their own engine Presto to WebKit named Blink.

²⁾ On April 3 2013 Google (Chrome version 28+) announces they are going to use the WebKit-based Blink engine.

³⁾ On December 6 2018 Microsoft (Microsoft Edge 79+ stable) announces they are going to use the WebKit-based Blink engine.