and how about systems that adapt their gui to the behavior of the specif user or of a group of similar users? (for example because the user agent sends an ID to the server that identifies the user as a blind person?) are these non conformant websites?
This success criterion applies to the logical order and default presentation of the content. This success criterion does not prohibit choices in adaptive interfaces; enabling adaptive interfaces and alternate presentations is a goal of WCAG2. However, the user still needs to initiate the change in order, even if the change is initiated by global actions such as selecting preferences in a user agent or using a user agent with adaptive behavior. We have added this explanation to the Intent Section of the How To Meet document.