It's like the browser's kernel. You can build different browsers on top of the same engine without having to write everything from scratch. Chrome, Opera, Edge

It's like the browser's kernel. You can build different browsers on top of the same engine without having to write everything from scratch. Chrome, Opera, Edge and Brave all share the same Blink/Webkit engine and there are a bunch of Firefox forks like Palemoon that share its Gecko engine. Servo is a new engine written from scratch that's progressively replacing Gecko in Firefox: https://wiki.mozilla.org/Quantum https://wiki.mozilla.org/Quantum