The Center for Universal Design defines universal design as "the design of products and environments to be usable by all people, to the greatest extent possible, without the need for adaptation or specialized design." See http://terrymorris.net/universaldesign for a video overview of universal design for the web.
Awareness of universal design by web developers has been steadily increasing. Web accessibility — often left until the last moment — has garnered increased attention and is now more typically integrated into the design and coding process. Accessible web sites, with alt text for images, heading tags used in an organized manner, text navigation links, and captions or transcriptions for multimedia are more easily used not only by visitors with disabilities but also by visitors using a mobile browser. An accessible web site may also help with search engine optimization.
Here are some resourcs to jumpstart your accessible design skillset: