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Yes, you can create an accessible site in Xoops. It is not easy. Most CMS systems you come across are not going to be easy to validate according to accessibilty standards.
The problem is our history, I mean among all CMS. When these systems were created, it was standard practice to use tables and such for layout. Even browsers can't agree on the standards, yet. So, don't expect the moon no matter which CMS or system you choose.
I have used probably every CMS around for one thing or another. I can tell you that XOOPS is going to be the best CMS to learn if you are attempting one for the very first time. This goes for installation, maintenance, and design.
A CMS uses modules or extensions, they all have their own treminology, for adding the various functions. Many of these are also not coded to generate standards compliant output.
When you design your main theme, you can make sure your "wrapper" is compliant, but cleaning up the content from modules and core functions can be a pretty big chore. It can be done, but it usually means writing a template for every module in most CMS systems.
Good luck.