Perception Through Equivalence

At 14 success criteria, the Perceivable principle requires a good deal of effort to meet. In addition to having the greatest number of success criteria, the Perceivable principle also contains subjective criteria, like providing text alternatives for images. The Perceivable principle is heavily rooted in the concept of providing equivalence in digital content. For example, providing a text description of imagery exposes that content to the non-sighted user's screen reader, thus allowing that user to perceive the content in an equivalent fashion to a sighted user. 

Guideline 1.1 Text Alternatives

Provide text alternatives for any non-text content so that it can be changed into other forms people need, such as large print, braille, speech, symbols or simpler language.

Providing alternative text descriptions is one of the most fundamental aspects of ICT accessibility. The most common use of alternative text description is in auditory expression announced by a screen reader to the non-sighted user. That same description can be useful to search engines as we define a taxonomy to non-text content. 

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Guideline 1.2 Time-based Media

Provide alternatives for time-based media.

Another form of non-text content that requires text descriptions comes in the form of time-based media formats: video and audio (both live and pre-recorded). Whether through captioning or transcription, this guideline provides equivalent access to the Deaf and hard-of-hearing user by describing video or audio content through the modality of text. 

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Guideline 1.3 Adaptable

Create content that can be presented in different ways (for example, simpler layout) without losing information or structure.

When content is formatted in such a way as to derive meaning from its visual appearance, there are a number of concerns that arise for diverse abilities. The visual presentation of a table, list, heading, or link, should also have an equivalent programmatic markup to communicate with user agents and assistive technologies. Similarly, when the order or presentation affects the meaning of content, that order should be presented programmatically to the user in a meaningful way. And finally, the use of spatial relationships, size, location, and orientation in the visual presentation should not be the only means of understanding or operating content. 

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Guideline 1.4 Distinguishable

Make it easier for users to see and hear content, including separating foreground from background.

This guideline will have the greatest impact to visual design as it instructs that color alone should not be used to convey information. An example would be the legend of a bar chart that describes data series through colors to associate the values graphed in the XY coordinates of the chart. In addition, there are minimums of contrast to adhere to that ensure low-sighted individuals can distinguish text content from foreground to background.

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