Us ability?
No, USE-ability. It's about giving people the means to effectively use and access the information that makes up a site. Regardless of the tools a visitor may use to access a site, the content needs to be available in some fashion. This may mean doing things like making multiple versions of the same site (IE optimized, mobile optimized, etc) or providing additional metadata that explains what things on the page are or do.
Horton and Lynch explain that designers can take a user-centered approach in designing our site in order to maximize usability or reach universal usability. As designers work on documents, they need to consider how a wide range of users may feel about a site or how the site performs for them. When working on a user-centered design for a site, the principles noted on pages 54 and 55 of Web Style Guide should be considered. These principles (from The Center for Universal Design at NC State's College of Design) include equitable use, flexibility in use, simple and intuitive use, and perceptible information. Although users will have some core knowledge and do some amount of work to browse a document, the aim is to create documents that people don't have to think about or workaround when browsing.