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. (Level AA)

Sufficient Techniques for 3.3.3 – Error Suggestion

Situation A: If a mandatory field contains no information:

• 1.G83: Providing text descriptions to identify required fields that were
  not completed

Situation B: If information for a field is required to be in a specific data
format:

• 1.G85: Providing a text description when user input falls outside the
  required format or values
• 2.G177: Providing suggested correction text
• 3.SCR18: Providing client-side validation and alert
  (Scripting)
• 4.SCR32: Providing client-side validation and adding error text via the
  DOM
  (Scripting)

Situation C: Information provided by the user is required to be one of a limited
set of values:

• 1.G84: Providing a text description when the user provides information
  that is not in the list of allowed values
• 2.G177: Providing suggested correction text
• 3.SCR18: Providing client-side validation and alert
  (Scripting)
• 4.SCR32: Providing client-side validation and adding error text via the
  DOM
  (Scripting)

Advisory Techniques for 3.3.3 – Error Suggestion

• G139: Creating a mechanism that allows users to jump to errors
• Making error messages easy to understand and distinguishable from
  other text in the Web page
• Validating form submissions on the server
• When mandatory information has not been provided, including descriptions
  or examples of correct information in addition to identifying the field as mandatory
• Repeating and emphasizing suggestions for correcting each input
  error in the context of its form field
• Providing a way for the user to skip from each item in a list of
  suggestions to its corresponding form field
• Providing additional contextual help for the form field requiring
  change
• Accepting input data in a variety of formats
• G199: Providing success feedback when data is submitted successfully

Techniques for providing suggestions to the user (Advisory)

• Providing a text description that contains information about the
  number of input errors, suggestions for corrections to each item, and instructions
  on how to proceed
• Providing a text description that contains suggestions for correction
  as the first item (or one of the first items) of content, or emphasizing this
  information in the content
• Displaying errors and suggestions in the context of the original
  form (for example, re-displaying a form where input errors and suggestions for
  correction are highlighted and displayed in the context of the original form)

HTML Techniques (Advisory)

• Providing “correct examples” for data and data formats
  as initial text in mandatory form fields
• Providing links to suggested correction text “close to”
  form fields, or providing the suggested correction text itself directly on the
  Web page “next to” form fields

Client-Side Scripting Techniques (Advisory)

• SCR18: Providing client-side validation and alert
  (Scripting)
• Providing client-side validation and adding error text via the DOM
• Calling a function from the submit action of a form to perform client
  side validation

ARIA Techniques (Advisory)

• ARIA2: Identifying required fields with the “required” property
  (ARIA)
• ARIA3: Identifying valid range information with the “valuemin”
  and “valuemax” properties
  (ARIA)

http://www.w3.org/TR/UNDERSTANDING-WCAG20/minimize-error-suggestions.html