The SILE Typesetter — Simon’s Improved Layout Engine
...Conceptually, SILE is similar to TeX—from which it borrows some concepts and even syntax and algorithms—but the similarities end there. Rather than being a derivative of the TeX family SILE is a new typesetting and layout engine written from the ground up using modern technologies and borrowing some ideas from graphical systems such as InDesign.
GSS is a JavaScript layout engine based on Grid Style Sheets (constraint CSS) using Cassowary constraint solver. It enables declarative relational layouts—expressing 'item A aligned to the right of item B'—and compiles these constraints into standard styles at runtime or build-time. Though experimental and labeled unstable, it showcases advanced layout paradigms beyond traditional CSS.