Josh Matthews (@lastontheboat) Research Engineer, Mozilla Firefox (Gecko) 9M lines of C/C++ Gecko's old style system 160K lines of C++ Gecko's new style system (Stylo) 85K lines of Rust What is a style system? A web browser is a pipeline. What is a style system? HTML parser: input: HTML source <html><head><style>...</style></head><body>...</body></html> output: tree of DOM nodes What is a style sy