People who are blind need text equivalents for the images used on the Web page because the assistive screen reader technology cannot obtain the information from the image. A screen reader announces the item so the individual knows where the focus is on the page. At that point, the individual presses the Enter key instead of "clicking" the mouse button.
Blindness involves a substantial, uncorrectable loss of vision in both eyes. To access the web, many individuals who are blind rely on screen readers. A Screen Reader is software that reads text on the screen and outputs this information to a speech synthesizer and/or refreshable Braille display.
Some people with blindness use text-base browsers such at Lynx, or voice browsers, instead of a graphical user interface browser plus screen reader. Additionally, they may use rapid navigation strategies such as tabbing through the headings or links on the Web pages rather than reading every word on the page in sequence.
Below are examples of accessibility barriers that people with blindness may encounter on a website:
- Images that do not have alt text.
- Complex images (e.g. graphs or charts) that is not adequately described.
- Video that is not described in text or audio.
- Tables that do not make sense when read serially (in a cell-by-cell or "linearized" mode).
- Frames that do not have “NOFRAME” alternatives or that do not have meaningful names.
- Forms that cannot be tabbed through in a logical sequence or that are poorly labeled.
- Browsers and authoring tools that lack keyboard support for all commands.
- Browsers and authoring tools that do not use standard applications programmer interfaces for the operating system they are based on.
- Non-standard document formats that may be difficult for their screen reader to interpret.
A person who has a visual disability may not find the mouse useful because it requires hand and eye coordination. Instead, the individual will navigate the Web page using only the keyboard.
Those who have low vision may need the assistance of a hardware or software magnifier to enlarge the text beyond simple font enlargement. If information is presented using any attribute by itself (for example, contrast, depth, size, location, or font), an individual with low vision might not detect the difference.
Magnification can reformat the location, change the contrast, or distort the size and fonts of the text and objects on the Web page. For those reasons, it is recommended to use multiple attributes. Here is an example from W3C: If both color and a fill pattern are used on different bars on a graph, they can be viewed in either color or black and white. Instead of using size attributes on the font element to denote a heading, the heading element should be used to correctly mark up a heading so that assistive technology can identify headings.
Here are some types of low vision:
- Poor Acuity – vision that is not sharp
- Tunnel Vision – can only see the middle of the visual field
- Central Field Loss – can only see the edges of the visual field
- Clouded Vision – vision is obscured
Individuals with low vision use the web with extra-large monitors, and/or increase the size of system fonts and images. Additionally, some use screen magnifiers or screen enhancement software or specific combinations of text and background colors, such as a 28-point white font on a black background, or choose certain typefaces that are especially legible for their particular vision requirements.
Here are some examples of accessibility barriers for persons with low vision:
- Web pages with absolute font sizes that do not change easily.
- Web pages that, because of inconsistent layout, is difficult to navigate when enlarged, due to loss of surrounding context.
- Web pages or images on Web pages, that has poor contrast, and whose contrast cannot be easily changed through user override of author style sheets.
- Imaged text that cannot be re-wrapped.
People who are colorblind benefit from good contrasting colors. If information is presented by color alone, an individual with colorblindness may miss the information.
Color blindness is a lack of sensitivity to certain colors. Common forms of color blindness include difficulty distinguishing between red and green, or between yellow and blue. Color blindness may also result in the inability to perceive any color.
Individuals with color blindness can use their own style sheets to override the font and background color choices of the author.
Here are some examples of accessibility barriers for individuals with color blindness:
- Color that is used as a unique marker to emphasize text on the site.
- Text that inadequately contrasts with background color or patterns.
- Browsers that do not support user override of authors’ style sheets.
Written by Debra Ruh of TecAccess