Accessibility Tutorial
Types of Disabilities and Associated Accessibility Issues
Visual Disabilities
There are several types of visual disabilities including partial or total blindness, inability to read text that is too small, color blindness, and eye-to-brain coordination, which can affect a user's ability to proceed from line to line. Users that are mostly or totally blind will likely use a screen reader to access a page, or perhaps a Braille device that converts text into Braille characters, while others will enlarge font size in their User Agent, generally a visual browser. Others will override the site style sheets and use their own high contrast color combinations that increase readability. Readers with eye-to-brain coordination problems might be unable to read long chunks of text if the columns are too wide, the paragraphs are too long, or the leading (space between lines) is too small.
Common problems for people with visual disabilities occur when they encounter images, image maps, or Flash animations that lack text alternatives. Users may also have an inability to associate elements or information. This can occur when labels are not associated correctly with their corresponding form elements, when the labels are not actually marked up as HTML labels but only appear to be so, or when form elements lack any associative text altogether, as when a search field has no label indicating what it is. There are many visual cues on a page to associate images with text or instructions with buttons, but for the non-sighted user, these associations might not be obvious. Apparent proximity does not suffice for accessibility.
After a failed attempt at page submission there may be an inability to recognize that an error has occurred because the only clues to indicate the error are visual. This scenario may arise when JavaScript is used on the client-side for form error checking. JavaScript correctly blocks the submission of bad data, but since the page is not being re-submitted, the screen reader may not be aware that the error message is now visible.
A color blind user may have an inability to differentiate text links from plain text if the links have no underlines or visual cues aside from color, in that they may appear as plain text. Forms that use a color—usually the color red—to indicate a required field, will not be easily understood by a color blind user unless there are other clues available, such as an asterisk with a key explaining its use.
A sighted user can easily move the eye around the page past the upper navigation to the main content. A blind user has no means to skip to main content unless one is specifically provided. Lack of such a mechanism often leads to impatience, fatigue, and even bailout. Consider for a moment having to sit through the reading of twenty or more links and page headings before getting to the meat of the page, and then having to listen to that for every successive page. Such a web experience promotes tedium.
Motor Disabilities
Many people with motor disabilities do not use a mouse to follow links. They may use a keyboard, voice recognition software, a mouth stick, or a head wand. Thus, common problems for these people include an inability to perform a task before a session timeout, an inability to submit a form because they browse with JavaScript turned off, or an inability to select or otherwise interact with an element because it is too small or too close to another element on the page. Consider the tedium of having to hit the tab key dozens of times to get to a particular link. Imagine a page like Amazon.com's home page. This is an extremely large, content-laden page with dozens of links without a means of easily skipping around. Imagine trying to tab to a link near the bottom of the screen. That could take some time and effort.
People with motor disabilities are also more likely to make a mistake, perhaps by clicking on the wrong link, or worse, submitting a form when they meant to cancel the submission. Mechanisms must be in place to allow for the correction of mistakes. This is a design issue as much as an accessibility issue.
Photo Epileptic Seizures
Photo epileptic seizures can be caused by flashing effects. This issue is particularly related to design because designers will likely provide all Flash assets. However, DHTML effects could cause similar problems, but I am unaware of anyone having documented this.
Cognitive Disabilities
Aiding users with cognitive disabilities such as memory loss, dyslexia, or a lack of problem solving skills, can mostly be obviated with good design, good usability, and clear content. But steps can be taken by a web developer to help, such as increasing font leading, or ensuring that columns of text are not too wide.
Hearing Disabilities
A deaf user will not have an ability to hear spoken text, and thus an alternative should be provided, such as a transcript, or video captioning. This can also apply to a user with Flash disabled.