Browser
Software that fetches and renders web pages. Contains: HTML parser, layout engine, JavaScript engine, CSS engine, network stack, rendering pipeline, devtools.
Three browser engines remain in 2025: Blink/Chromium (Chrome, Edge, Brave, Opera, Arc), Gecko (Firefox), WebKit (Safari, iOS). Servo (Mozilla 2012, then Linux Foundation) is a Rust-based experimental engine — recently revived. A modern browser is often 30M+ lines of code — among the most complex consumer software. Resource-hungry (RAM!) but necessary for "web as platform".