RmlUi-based UI for Unreal: markup and stylesheets instead of widget graphs, distance-field text and vector art sharp at any size, and a preview tool that lays out identically to the engine.
Read Start here in order if you are new. Everything after it stands alone.
RmlUi is not a browser and RCSS is not CSS. Valid-looking CSS parses and then does nothing: background-image, text-shadow, clamp(), 120ms, rgba(0,0,0,0.5). None of those warn.
Two habits cover it:
Run with --strict. Nearly every RCSS mistake is a warning rather than an error, and a check that only looks at errors passes a broken stylesheet.
When something does nothing, look it up in UI_CONVENTIONS.md before debugging it; it is usually on the banned list.
Stacked captions: several speakers at once, each line on its own clock, a name and colour per speaker, sounds set apart from speech. Why the sound does not decide how long one holds, and why a line has to be held in the list to animate away.
The whole vocabulary in one build: an oval screen, a text field, a draggable window with a bin and an animated close, onto a curved surface with scan lines.
Items that span several cells: grid lines from gradients, a ghost that shows whether a drop fits, locked cells, and one set of commands for mouse, keyboard and pad.
A first-person visor with no art at all: a helmet made of one border, curvature without 3D transforms, two components trailing the camera and the stride, and baking the aperture glow so it stops costing a render target a frame.
A character in a profile frame and meshes in inventory cells: isolating a capture without a staging area, the premultiply trap, and one rig feeding a page of targets.
Standing in fire, taking a hit, running on low health: an inset box-shadow is the whole effect, states cross-fade while events animate, then bake the lot to textures, because a full-screen layer costs a render target every frame.
A pixel-art character on an angled HUD panel that walks, runs, jumps, ducks and skids with the player: packing frames, a bound <flipbook>, and picking the clip from character movement.
Authoritative for capability. What RCSS supports, what is banned, and what silently does nothing. Read it before authoring anything real: RmlUi is close enough to CSS to mislead.