Web browsers are astoundingly complicated. They generally contain multiple JIT compilers, a database engine, sophisticated rendering code for 2d and 3d graphics and fonts, video and audio codecs, support for a variety of network protocols, graphical debuggers and development tools... the list goes on. And they expose all this to arbitrary applications downloaded from the internet while attempting (and mostly succeeding) to maintain your security and privacy and remain backwards compatible with 20+ years of legacy content. At a target frame rate of 60 fps.

Browsers are, when you think about it, pretty damn amazing.