In the beginning of the web, the site were based with largely text. But, things are different now. Today it is possible to navigate entire sites by reading no more than a handful of words. Users have grown tremendously to colorful image.
But when users cannot see these images, navigation is far more difficult where the site may be totally inaccessible and this would be the disadvantage. However, information content is dispersed among words, picture, sounds and video. Good layout makes for inviting reading where the reader can identify the text and it may read words out of sequence since sentences have been chopped up to improve the page layout.
So how is one to get the content and hopefully the context of information on today's sites? In order to get around these issues, we need to design web pages with accessibility in mind from the beginning. And all of the content needs to be accessible through multiple channels.
At the very least the content needs to be text-based, and current web page standards have taken this into consideration. When developers place pictures onto a web page, they may be asked for alternate text for these images. This is not where developers name or comment on the picture; this is where they describe what is in the picture. They must imagine they are trying to describe the image to a friend over the phone.
Other disabilities benefit from this duplication of content across multiple channels. A deaf person can still see the video and now can read a narration. And persons with cognitive disabilities, especially those that limit attention span, can watch a video, see the text, and hear the text all at once. Information is presented with multiple channels and these channels increase the ability to concentrate on that information by blocking out other input.
By following the idea of universe recommendation for one disability, a web site becomes accessible to a broad range people.