There are a lot of things we take for granted when designing and viewing Web pages. For instance, browsers are very forgiving of bad code, and they can make a page with bad code look better than it should based solely on the code. Can all browsers and all assistive technologies handle bad code as gracefully? We usually assume that the viewers to our Web site will understand the language it’s in, and we assume that if they use an assistive technology that it will be able to easily understand the language too. What if a user whose screen reader is set to read Spanish comes to an English Web page? How will the screen reader know how to speak the words?
The point is there are some assumptions we often make that we probably should not make because we don’t know who will be accessing our site or how they will be accessing it.