Many have argued about whether a web designer has the knowledge to do search engine optimization (SEO). I think before you make that decision you should look at their past work. If they are just learning SEO you might be in trouble but I have met some designers who are very competent at SEO. The real question is if they have the time to do the work.

Tom Shivers makes the case that only a specialist can do the job here.

Health and Human Services has issued updated usability guidlines for websites. These guidelines are intended to provide users with disabilities access to the same information that is available to other users.

Read all the guidelines here(beware it is a large pdf).