Blink has a younger code base that’s easier to build on. Gecko has been around since the early 90s and has some ancient evils lurking deep within.
They both are of very similar age actually. The old Netscape rendering engine originated in the early 90s, but Gecko was a rewrite from scratch that was first used in a browser in 1998.
Blink is based on KHTML which is based on khtmlw, which was written at some point in the mid-90s, but as well saw a complete rewrite in 1999.
anonymoose