Here's @alcinnz's Odysseus (Flatpak build with libunity manually patched out) next to GNOME Web. Despite some theming issues (oversized and missing icons, that misplaced menu, flashing autocompletions, misaligned text... — I've seen the same with Tootle, another Elementary app) it looks quite native and actually works (in my brief testing) well. With a little polish it could be a viable alternative to Web.
...not that I can switch away from Chrome 😞
So here's something pretty cool I wrote ≈ 2 years ago.
It can do windowing (with CSD, resizing and proper cursors), boxes (widgets that multiplex several child widgets), buttons (render a border, emit a callback when clicked) and labels (render text). Widgets can define their sizing constraints (min width, min height) & it's propagated through the tree.
Not much else.
So here's a thing