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There are several types of disabilities that impact computer use:
About 10% of people suffer from some form of colorblindness. In a well-designed user interface, color should not be the only way of distinguishing between different pieces of information.
Some people may not be able to use one of the input devices, the mouse or the keyboard. Therefore it is important for software functions to be accessible using both of them. Keyboard shortcuts and mouse gestures are ways to achieve this. More specialized solutions like on-screen keyboards and alternate input devices like joysticks and trackballs are also available.
The astrophysicist Stephen Hawking is a famous example of a person suffering from motor disability. He uses special software that allows him to control his computer using the movement of only two fingers.
On the World Wide Web, the W3C has produced specific guidelines for accessibility. The Cascading Style Sheets system has been devised with this in mind, since it gives the reader full control over the appearance of the page.
As an example, most web browsers have an option to ignore the font size specified in a webpage, so that the user can circumvent a small font forced upon him or her by a webpage author. However, sometimes a webpage author fails to take into account that users may want to apply such an option and designs a webpage such that applying this option gives poor results, such as too small a distance between lines, disabled scrolling even though texts do not fit in assigned spaces, overlapping texts, etc.