I like the work that Gov UK do in terms of accessibility, changes they make, and how they explain it. Not just from a compliance point of view, but a lot of thought goes into their design too. The first time I had to file my taxes (HMRC site), I was terrified, but their site guided me through the process, with clear divisions, sections, guidance along the way. In all of this their design decisions really shines, and it makes a lot of sense; dealing with legalese and paperwork is daunting, the last thing a user should be doing is having to 'navigate' your website.