Often, you don't even need to lift a finger, design-wise or implementation-wise, to have good accessibility. The system-wide or toolkit defaults are usually quite accessible right out of the box. macOS's horrible disappearing scrollbars being a huge exception.

Most of the terrible designs and accessibility problems stem from software going out its way to write custom controls or force controls to look and behave in a non-default way. It's not an afterthought--people are deliberately adding code to make their software worse.