I find it difficult to separate the issues of accessibility and usability without looking first at some definitions:
Branjnik 2000 as cited in Seale (pp84) defines usability as “the effectiveness, efficiency and satisfaction with which the specified users achieve specified goals in particular environments”
Seale (pp29) defines accessibility as “removing bariers to participation and engagement in the online experiences and the degree to which someone can acess an online resource regardless of their disability, technology or environment”
These definitions do little to help me clarify significant different outcomes for accessibility and useibility as the definition of accessibility is about “removing bariers to participation and engagement…”, and poor usability is one of those barriers to participation and engagement. While the usability, is about the “effectiveness, efficiency and satisfaction” of “achieving specified goals” which is not possible without accessibility.
It has been argued that accessibility and usability are signifficantly different as website can met accessibility standards without being usable (Thacher 2004 as cited in seale pp86) However I suggest that this mealy highlights problems with the standards used as the website would not be truly accessible if it was not usable.
I understand that the words readability, usability and accessibility mean different things however I feel that the outcomes are so intertwined that trying to seperate out for the purpose of creating guidelines, would lead to more barriers to good (i.e. accessable, readable and usable) design.