• Images that do not have alternative text
• Complex images (e.g., graphs or charts) that are not adequately described
• Video that is not described in text
• Tables that do not make sense when read serially (in a cell-by-cell or "linearized" mode)
• Forms that cannot be tabbed through in a logical sequence or that are poorly labeled

Low vision

There are many types of low vision, for instance 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.

Some people with low vision use extra-large monitors, and increase the size of fonts and images. Others use screen magnifiers or screen enhancement software. Some individuals use specific combinations of text and background colors, such as a 24-point bright yellow font on a black background, or choose certain typefaces that are especially legible for their particular vision requirements.

Barriers that people with low vision may encounter on the Web:

• Web pages with absolute font sizes that do not change (enlarge or reduce) easily
• Web pages that, because of inconsistent layout, are difficult to navigate when enlarged, due to loss of surrounding context
• Web pages, or images on Web pages, that have poor contrast, and whose contrast cannot be easily changed through user override of author style sheets
• Text presented as images, which prevents wrapping to the next line when enlarged
• Also many of the barriers listed for blindness, above, depending on the type and extent of visual limitation

Color blindness

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. Sometimes color blindness results in the inability to perceive any color.

Barriers that people with color blindness may encounter:

• Color that is used as a unique marker to emphasize text on a Web site
• Text that inadequately contrasts with background color or patterns
• Browsers that do not support user override of authors' style sheets

Hearing Impairment

Deafness

Involves a substantial uncorrectable impairment of hearing in both ears. Some deaf individuals' first language is a sign language, and they may or may not read a written language fluently, or speak clearly.

To use the Web, many people who are deaf rely on captions for audio content. They may need to turn on the captions on an audio file as they browse a page; concentrate harder to read what is on a page; or rely on supplemental images to highlight context.

Barriers that people who are deaf may encounter:

• Lack of captions or transcripts of audio on the Web, including webcasts
• Lack of content-related images in pages full of text, which can slow comprehension for people whose first language may be a sign language instead of a written/spoken language
• Lack of clear and simple language
• Requirements for voice input on Web sites

Hard of hearing

A person with a mild to moderate hearing impairment may be considered hard of hearing.

To use the Web, people who are hard of hearing may rely on captions for audio content and/or amplification of audio. They may need to toggle the captions on an audio file on or off, or adjust the volume of an audio file.

Barriers encountered on the Web can include:

• Lack of captions or transcripts for audio on the Web, including webcasts

Physical disabilities

Motor disabilities

Motor disabilities can include weakness, limitations of muscular control (such as involuntary movements, lack of coordination, or paralysis), limitations of sensation, joint problems, or missing limbs. Some physical disabilities can include pain that impedes movement. These conditions can affect the hands and arms as well as other parts of the body.

To use the Web, people with motor disabilities affecting the hands or arms may use a specialized mouse; a keyboard with a layout of keys that matches their range of hand motion; a pointing device such as a head-mouse, head-pointer or mouth-stick; voice-recognition software; an eye-gaze system; or other assistive technologies to access and interact with the information on Web sites. They may activate commands by typing single keystrokes in sequence with a head pointer rather than typing simultaneous keystrokes ("chording") to activate commands. They may need more time when filling out interactive forms on Web sites if they have to concentrate or maneuver carefully to select each keystroke.

Barriers that people with motor disabilities affecting the hands or arms may encounter include:

• Forms that cannot be tabbed through in a logical order
• Browsers and authoring tools that do not support keyboard alternatives for mouse commands
• Time-limited response options on Web pages

Speech disabilities

Speech disabilities - Speech disabilities can include difficulty producing speech that is recognizable by some voice recognition software, either in terms of loudness or clarity.

To use parts of the Web that rely on voice recognition, someone with a speech disability needs to be able to use an alternate input mode such as text entered via a keyboard.

Barriers that people with speech disabilities encounter on the Web can include:

• Web sites that require voice-based interaction and have no alternative input mode

Cognitive and neurological disabilities

Visual and auditory perception

Individuals with visual and auditory perceptual disabilities, including dyslexia, sometimes called "learning disabilities" and dyscalculia may have difficulty processing language or numbers. They may have difficulty processing spoken language when heard ("auditory perceptual disabilities"). They may also have difficulty with spatial orientation.

To use the Web, people with visual and auditory perceptual disabilities may rely on getting information through several modalities at the same time. For instance, someone who has difficulty reading may use a screen reader plus synthesized speech to facilitate comprehension, while someone with an auditory processing disability may use captions to help understand an audio track.

Barriers that people with visual and auditory perceptual disabilities may encounter on the Web can include:

• Lack of alternative modalities for information on Web sites, for instance lack of alternative text that can be converted to audio to supplement visuals, or the lack of captions for audio
• Lack of clear writing or instructions

Attention Deficit Disorder

To use the Web, an individual with an attention deficit disorder may need to turn off animations on a site in order to be able to focus on the site's content.

Barriers that people with attention deficit disorder may encounter on the Web can include:

• Distracting visual or audio elements that cannot easily be turned off
• Lack of clear and consistent organization of Web sites

Mental health disabilities

Individuals with mental health disabilities may have difficulty focusing on information on a Web site, or difficulty with blurred vision or hand tremors due to side effects from medications.

To use the Web, people with mental health disabilities may need to turn off distracting visual or audio elements, or to use screen magnifiers.

Barriers can include:

• Distracting visual or audio elements that cannot easily be turned off
• Web pages with absolute font sizes that do not enlarge easily

Seizure disorders

Some individuals with seizure disorders, including people with some types of epilepsy (including photo-sensitive epilepsy), are triggered by visual flickering or audio signals at a certain frequency.

To use the Web, people with seizure disorders may need to turn off animations, blinking text, or certain frequencies of audio. Avoidance of these visual or audio frequencies in Web sites helps prevent triggering of seizures.

Barriers can include:

• Use of visual or audio frequencies that can trigger seizures