Kudos to WordPress for doing well in Joe Clark’s assessment of WordPress against the Authoring Tool Accessibillity Guidelines. The main complaint seems to be against the WordPress toolbar in the Write/Edit page in the admin, which I would argue needs further development. After all, there are cases where you want i elements instead of ems: for example, any proper title that you choose to italicize really isn’t something that you’re emphasizing; you’re just using italic type to alert the reader to the presence of a proper title.
The easiest way to summarize Clark’s finding is this: WordPress’s core commitment to producing standards-compliant markup makes it generally very accessible. The WP development crew should read Clark’s assessment, feel a sense of pride, and heed the minor suggestions and constructive criticisms provided.