Before you start picking up a library like this, keep in mind that no immediate-mode GUI library I'm aware of (including this one from what I can see in the source code) has support for assistive technologies such as screen readers.
To them the whole UI is one big black box, so your program is completely useless to anyone with a wide range of disabilities.
Do operation systems have APIs, so the program can send them what is on the screen? I mean just description like: There is the button with title "Push me" in the rectangle(50, 50, 100, 70), etc.
Or are there any "screen reader" libraries?
Aren't you then trying to bridge an immediate mode API (rendering controls) and a retained mode API (assistive tech).
Yes. This has influenced the design of my AccessKit [1] project. With AccessKit, the application (or GUI toolkit) pushes tree updates to the platform adapter, which maintains a complete tree that it uses to implement the platform-specific accessibility API. Each application-supplied tree update can be a full tree or just the nodes that have changed. So an immediate-mode GUI can push a full tree every frame. There just has to be some way of keeping node IDs stable across frames.