Understandable: Making Content Clear and Comprehensible

The third principle of WCAG, Understandable, requires that information and the operation of the user interface must be understandable. This means users should be able to comprehend the content and learn how to use the interface without undue confusion or cognitive load.

A clear, well-structured document or interface, symbolizing understandable information flow. — Content and interface operations must be clear and comprehensible.

This principle is vital for users with cognitive or learning disabilities, but clear and predictable content benefits everyone. It involves making text readable, ensuring predictable behavior of web elements, and helping users avoid and correct mistakes.

Guideline 3.1: Readable

Make text content readable and understandable. This involves considerations of language, complex terminology, and abbreviations.

Language of Page: The default human language of each Web page can be programmatically determined (e.g., using the lang attribute on the <html> element).
Language of Parts: The human language of each passage or phrase in the content can be programmatically determined (e.g., using the lang attribute on specific elements if they are in a different language than the main page).
Unusual Words: A mechanism is available for identifying specific definitions of words or phrases used in an unusual or restricted way, including idioms and jargon.
Abbreviations: A mechanism for identifying the expanded form or meaning of abbreviations is available.
Reading Level: When text requires reading ability more advanced than the lower secondary education level after removal of proper names and titles, supplemental content, or a version that does not require reading ability more advanced than the lower secondary education level, is available. (Level AAA)

Clear language is essential. For instance, Introduction to Natural Language Processing (NLP) explores how machines can understand and process human language, a field relevant to making digital content more understandable.

Guideline 3.2: Predictable

Make Web pages appear and operate in predictable ways. When users can predict how interface elements will behave, they can navigate and interact with more confidence and less cognitive effort.

On Focus: When any user interface component receives focus, it does not initiate a change of context.
On Input: Changing the setting of any user interface component does not automatically cause a change of context unless the user has been advised of the behavior before using the component.
Consistent Navigation: Navigational mechanisms that are repeated on multiple Web pages within a set of Web pages occur in the same relative order each time they are repeated, unless a change is initiated by the user.
Consistent Identification: Components that have the same functionality within a set of Web pages are identified consistently.

A consistent navigation menu and interface elements across different hypothetical screens. — Predictable interfaces reduce confusion and improve usability.

Guideline 3.3: Input Assistance

Help users avoid and correct mistakes. Providing clear instructions, feedback, and error recovery mechanisms is crucial for usability, especially for users who may be prone to errors or have difficulty correcting them.

Error Identification: If an input error is automatically detected, the item that is in error is identified and the error is described to the user in text.
Labels or Instructions: Labels or instructions are provided when content requires user input.
Error Suggestion: If an input error is automatically detected and suggestions for correction are known, then the suggestions are provided to the user, unless it would jeopardize the security or purpose of the content.
Error Prevention (Legal, Financial, Data): For Web pages that cause legal commitments or financial transactions for the user to occur, that modify or delete user-controllable data in data storage systems, or that submit user test responses, at least one of the following is true: Reversible, Checked, Confirmed.

Effective input assistance is similar to how Prompt Engineering guides AI to produce desired outputs, by providing clear instructions and constraints.

A web form showing clear error messages and suggestions for correcting input. — Assisting users to avoid and correct mistakes is key to an understandable experience.

By ensuring your content is understandable, you cater to a wider audience and improve the overall user experience. The final WCAG principle, Robust, focuses on compatibility with current and future technologies.