<p>Visual disabilities range from mild or moderate vision impairments in one or both eyes ("low vision" or "partial sight"), to substantial and uncorrectable loss of vision in both eyes ("blindness"). Some people have reduced or lack of sensitivity to certain colors ("color blindness"), or increased sensitivity towards excessive brightness in colors. These variations in perception of colors and brightness can be independent of the visual acuity.</p>

<p>Visual impairments include disorders that affect the central vision acuity, the field of vision, color perception, or binocular visual function. The American Medical Association defined legal blindness as visual acuity not exceeding 20/200 in the better eye with correction, or a limit in the field of vision that is less than a 20 degree angle (tunnel vision). Tumors, infections, injuries, retrolental fibroplasis, cataracts, glaucoma, diabetes, vascular impairments, or myopia may cause legal blindness.</p>

<p>Visual disabilities vary widely. Some users may use Braille and others do not. Users with some vision use a variety of accommodations, equipment and compensatory strategies. These may include enlarged print and/or magnifiers, speech input/output software programs, tape recorders and test readers.</p>

<p class="environment">More about visual disabilities</p>

<p>People with visual disabilities typically rely on changing the presentation of web content into forms that are more usable for their particular needs. For example by:</p>

<ul>

<li>Enlarging or reducing text size and images</li>

<li>Customizing settings for fonts, colors, and spacing</li>

<li>Listening to text-to-speech synthesis of the content</li>

<li>Listening to audio descriptions of video in multimedia</li>

<li>Reading text using refreshable braille (small dots that are raised and lowered to display characters that are read by scanning over the raised dots using the fingertips)</li>

</ul>

<p>For these web browsing methods to work, developers need to ensure that the presentation of web content is independent of its underlying structure, and that the structure is correctly coded so that it can processed by software and presented in different ways. For instance, some people do not see the content and need lists, headings, tables, and other page structures to be properly coded so that they can be identified by software.</p>

<p>Some people are only seeing small portions of the content at a time or are perceiving the colors and design differently. Some people are using customized fonts, colors, and spacing to make the content more readable, or they are navigating through the content using keyboard only because they cannot see the mouse-pointer. An accessible design supports different presentations of the web content, and different ways for interaction.</p>

<p class="environment">Examples of visual disabilities</p>

<ul>

<li>Blindness - substantial, uncorrectable loss of vision in both eyes</li>

<li>Color blindness - includes difficulty distinguishing between colors such as between red and green, or between yellow and blue, and sometimes inability to perceive any color</li>

<li>Deaf-blindness - substantial, uncorrectable visual and hearing impairments</li>

<li>Low vision - includes poor acuity (vision that is not sharp), tunnel vision (seeing only the middle of the visual field), central field loss (seeing only the edges of the visual field), and clouded vision</li>

</ul>

<p class="environment">Examples of barriers for people with visual disabilities</p>

<ul>

<li>Images, controls, and other structural elements that do not have equivalent text alternatives</li>

<li>Text, images, and page layouts that cannot be resized, or that lose information when resized</li>

<li>Missing visual and non-visual orientation cues, page structure, and other navigational aids</li>

<li>Video content that does not have text or audio alternatives, or an audio-description track</li>

<li>Inconsistent, unpredictable, and overly complex navigation mechanisms and page functions</li>

<li>Text and images with insufficient contrast between foreground and background color combinations</li>

<li>Websites, web browsers, and authoring tools that do not support use of custom color combinations</li>

<li>Websites, web browsers, and authoring tools that do not provide full keyboard support</li>

</ul>