I agree GNOME is resource heavy however that has nothing to do with Javascript being involved. The James Web Telescope uses Javascript for some of its core functionality (specifically managing its science modules), does that make it a web browser? I personally don’t like GNOME either, but most of it is written in C, it has its own GUI library which is written in C. The Javascript code likely just is used to simplify calling the underlying C functions and CSS is used for customizing the actual UI elements.