We had to retrofit some of the WCAG functionality into our product. Possible but better to have it as a requirement up front. The thing is, modern browsers are really good about accessibility so you gotta go out of your way to mess it up.
In my opinion it's like having ramps for wheelchairs. Not the first thing that needs to go up, but it should be soon. If you don't design your building with the ramp in mind you usually end up with an ugly design.