One clearly defined starting point is exposing any custom controls to accessibility APIs that are used to enumerate and interact (simulated mouse actions, reading the text, etc) with controls on the screen. Both scripting tools and screen readers make use of these. Built-in controls already have the enumeration and interaction feature and don't need additional code, but custom controls may not have that.

In the MicroUI example here, there are buttons and text labels and other kinds of controls, but no ability for an outside process to enumerate or interact with the controls. Any program will just set a single giant window with no text and no controls inside. Accessibility software can still hook text output APIs, but not if it also uses custom font rendering.

Anyway, the Windows accessibility API is just implementing a few COM objects, and COM (other than the specific ABI used for storing the vtable and function call convention) is not necessarily specific to Win32.

This is one of the reasons why web technology is so popular and persistent. You get almost all of that for free as long as you use semantic HTML.

> sad that the world is so hung up on unicode

It is sad that the world is hung up on enabling 2/3rds of the world population to read and write text! If only the entire world catered to America. Nobody should ever speak anything other than English, honestly.

Being accessible to the intended users always matters. If you think it doesn't, that probably means it's currently accessible to those users (or that those who are it is inaccessible to have filtered themselves out, and are no longer users).

For example - in your debug UI, colorblind-friendly colors don't matter, until you hire your twelfth member of the team, who struggles to tell red and green apart.

The point of a UI library is to interface with users. If it totally fails to interface with a subset of users then it is obviously deficient to some degree. It is callous and foolish to dismiss offhand users who rely on assistive technologies. You probably have a poor idea of who they are and how many people we’re talking about. You never know when you or someone you care about will become one of them, even temporarily. You never know how far your software will reach when you write it.