mikke89/rmlui ? reverse-engineered prompt

Build me a small C++ UI library project like RmlUi, with HTML and CSS like files that can be turned into an in app interface for games or desktop apps.

I want it to support common controls like buttons, sliders, dropdowns, text inputs, layouts, animation, transforms, styling, and a way to hook into my own renderer and input handling. It should be lightweight, cross platform, and not depend on a full browser. Please include a few sample screens so I can see how to use it, plus a simple build setup with CMake. If there are common backend options for rendering or windowing, wire in one sensible default and make it easy to swap later.

Also include the basic developer docs or comments needed to show how to create a UI context, load a document, update it in a loop, and render it. Look up current docs online if you need to.