There are tons of different disabilities both permanent and temporary. Someone could have a broken arm and incapable of using a mouse or someone could have a more permanent reason for not having use of the limb. It can be a visual, auditory, physical or cognitive disability. Under the ADA all levels of disability are covered. Visual impairment from color blindness and difficulty seeing all the way to complete blindness. Seizures, impaired mobility....everything. So a site has to be completely usable with a keyboard only because many people can't use a mouse properly. The colors have to meet contrast ratios to make them easy to see for people with color blindness. Images have to be coded properly to describe what the image is to people using a screen reader. Many times people like to say "Well those type of people wouldn't be at my website" but that's not an excuse to not allow them the ability to use it. It would be like saying I am not going to have wheelchair access to my store because they wouldn't want to shop here. On top of that there are so many situations where normal users can benefit by having an inclusive site. I may be able to use a mouse no problem but maybe it got damaged and all I have is a keyboard. If I am using a mobile device in bright sunlight those contrast ratios for color blind people will help the page be more visible under those harsh conditions.
Sorry to be long winded it's my job but also having an inclusive web is just something that I am passionate about.
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