The levels of conformance are the hierarchical tiers used in the Web Content Accessibility Guidelines (WCAG) to define how successfully web content meets established accessibility standards.
WCAG 2.2 defines three main levels of conformance:

1. Level A (Minimum Level): This is the lowest level of conformance. To achieve Level A conformance, a web page must satisfy all Level A success criteria, or a conforming alternate version must be provided.
    ◦ For example, meeting Level A requirements includes ensuring all non-text content has a text alternative (Success Criterion 1.1.1) and that audio playing automatically for more than three seconds has a mechanism available to pause or stop it (Success Criterion 1.4.2).
2. Level AA (Enhanced Level): This level requires a higher degree of accessibility than Level A. To achieve Level AA conformance, the web page must satisfy all the Level A and Level AA success criteria, or a Level AA conforming alternate version must be provided.
    ◦ WCAG 2.2 includes several new criteria at Level AA, such as Focus Not Obscured (Minimum) (2.4.11) and Dragging Movements (2.5.7).
3. Level AAA (Highest Level): This is the highest level of conformance. To achieve Level AAA conformance, the web page must satisfy all the Level A, Level AA, and Level AAA success criteria, or a Level AAA conforming alternate version must be provided.
    ◦ The WCAG 2.2 document notes that Level AAA conformance is generally not recommended as a policy requirement for entire sites because it is not possible to satisfy all Level AAA success criteria for some types of content.
    ◦ Examples of Level AAA requirements include Extended Audio Description (1.2.7) and Keyboard (No Exception) (2.1.3).
Key Details on Conformance:
• Structure: The WCAG success criteria, which determine conformance, are organized under four principles: Perceivable, Operable, Understandable, and Robust.
• Testing: The WCAG success criteria are designed as testable statements. Achieving conformance means that the content needs to meet these success criteria.
• Encouragement: While conformance is only achieved at the stated levels (A, AA, AAA), authors are encouraged to report any progress made toward meeting success criteria from levels beyond the achieved level of conformance.
• WCAG Versions: All versions of WCAG 2 (2.0, 2.1, and 2.2) utilize this three-tiered conformance model. Content that conforms to the latest version, WCAG 2.2, also conforms to WCAG 2.1 and WCAG 2.0 (it is "backwards compatible").
• PDF Accessibility: Separate from WCAG, accessible PDFs are documents that must meet the requirements specified in the ISO-standardized subset of PDF targeted at accessibility, called PDF/UA. Tools like axesPDF can validate the accessibility of PDF documents according to PDF/UA, WCAG, and Section 508.
These levels ensure that web accessibility requirements can be implemented progressively, from baseline requirements (A) to comprehensive enhancements (AAA)

Level A

The Level A requirements cover the following success criteria, organized by the four main principles of WCAG:

Principle 1: Perceivable
Content must be presented in ways users can perceive. Key requirements include text alternatives for non-text content, alternatives for time-based media (like transcripts for audio/video), captions for prerecorded audio in synchronized media, and audio descriptions or media alternatives for prerecorded video. Additionally, color cannot be the sole means of conveying information, and mechanisms to control audio that plays automatically are required. 

Principle 2: Operable
User interfaces and navigation must be operable. This principle requires all functionality to be accessible via keyboard without timing constraints, prevents keyboard focus from being trapped, and provides options for disabling or modifying character key shortcuts. Users must be able to adjust or turn off time limits unless essential, and pause or hide moving content. Content should not flash more than three times per second. Requirements also include mechanisms to bypass repeated content, descriptive page titles, logical focus order, and clear link purposes. For pointer-based interactions, functionality using multipoint or path-based gestures must also be achievable with a single pointer, pointer cancellation mechanisms are necessary, and the accessible name of a component must include its visual label. Functionality triggered by motion must have alternative controls and the motion should be disableable. 

Principle 3: Understandable
Information and interface operation must be understandable. This includes programmatically determining the language of the page, ensuring that components receiving focus or changing settings don't cause unexpected context changes, and providing consistent help mechanisms across pages. Input errors must be identified and described in text, and labels or instructions should be provided for user input. Previously entered information should not be unnecessarily required again. 

Principle 4: Robust
Content must be robust for various user agents, including assistive technologies. This principle requires valid markup and programmatically determinable name, role, and value for UI components. Status messages should be programmatically determinable without needing focus.

Level AA

The Level AA requirements, in addition to all Level A criteria, include:

Principle 1: Perceivable 

  • Captions for Live Content (SC 1.2.4): Provide captions for all live audio in synchronized media.
  • Audio Description (Prerecorded) (SC 1.2.5): Include an audio description for all prerecorded video in synchronized media.
  • Contrast (Minimum) (SC 1.4.3): Ensure text and images of text have a contrast ratio of at least 4.5:1 (or 3:1 for large text).
  • Resize Text (SC 1.4.4): Allow text to be resized up to 200% without losing content or functionality.
  • Images of Text (SC 1.4.5): Use text instead of images of text unless essential or customizable.
  • Reflow (SC 1.4.10): Content should not require horizontal scrolling at a width of 320 CSS pixels.
  • Non-Text Contrast (SC 1.4.11): User interface components and graphics need a minimum contrast ratio of 3:1.
  • Text Spacing (SC 1.4.12): Content and functionality should remain usable when users adjust text spacing.
  • Content on Hover or Focus (SC 1.4.13): Content appearing on hover or focus must be dismissible, hoverable, and persistent. 

Principle 2: Operable

  • Orientation (SC 1.3.4): Content should not be restricted to a single display orientation unless necessary.
  • Focus Not Obscured (Minimum) (SC 2.4.11): At least a portion of a focused component must be visible and not hidden by other content.
  • Multiple Ways (SC 2.4.5): Provide more than one way to find web pages within a set.
  • Headings and Labels (SC 2.4.6): Ensure headings and labels are descriptive.
  • Focus Visible (SC 2.4.7): Keyboard focus must have a clearly visible indicator.
  • Dragging Movements (SC 2.5.7): Offer an alternative single-pointer method for actions requiring dragging.
  • Target Size (Minimum) (SC 2.5.8): Interactive targets should be at least 24 by 24 CSS pixels, with some exceptions. 

Principle 3: Understandable

  • Identify Input Purpose (SC 1.3.5): Programmatically determine the purpose of form input fields using autocomplete.
  • Consistent Navigation (SC 3.2.3): Navigation repeated on multiple pages must be in the same relative order.
  • Consistent Identification (SC 3.2.4): Components with the same functionality should be identified consistently.
  • Error Suggestion (SC 3.3.3): Provide suggestions for correcting input errors unless it compromises security.
  • Error Prevention (Legal, Financial, Data) (SC 3.3.4): For significant transactions or data submission, allow users to reverse, correct, or confirm information.
  • Accessible Authentication (Minimum) (SC 3.3.8): Offer an alternative or help mechanism for authentication methods that rely on cognitive tests. 

Principle 4: Robust

  • Status Messages (SC 4.1.3): Ensure status messages can be programmatically determined by assistive technologies without receiving focus.

Level AAA

Achieving Level AAA can be challenging for all content, so it is often targeted for specific, essential sections of a website.
 
Principle 1: Perceivable

  • Audio Description or Media Alternative (Prerecorded) (SC 1.2.5): Requires an extended audio description for prerecorded synchronized media, or a full media alternative (like a text transcript including all visual and auditory information).
  • Sign Language (Prerecorded) (SC 1.2.6): A sign language translation must be provided for all prerecorded synchronized media with audio.
  • Audio Description (Live) (SC 1.2.7): For live synchronized media, an audio description is required.
  • Media Alternative (Prerecorded) (SC 1.2.8): A full alternative (text transcript) is needed for all prerecorded synchronized media.
  • Audio-only (Live) (SC 1.2.9): A live text alternative (like a real-time transcript) is required for all live audio-only content.
  • Identify Purpose (SC 1.3.6): The purpose of user interface components, icons, and regions must be programmatically determinable for all content.
  • Contrast (Enhanced) (SC 1.4.6): Text and images of text must have a contrast ratio of at least 7:1 (or 4.5:1 for large text).
  • Low or No Background Audio (SC 1.4.7): Prerecorded audio primarily containing speech must have no or very low background noise (at least 20 decibels lower than the speech).
  • Visual Presentation (SC 1.4.8): Provide mechanisms for users to adjust visual presentation of text blocks (e.g., width under 80 characters, no justification, 1.5 line spacing, 1.5 paragraph spacing).
  • Images of Text (No Exception) (SC 1.4.9): Text is used instead of images of text everywhere, including logos. 

Principle 2: Operable

  • Keyboard (No Exception) (SC 2.1.3): All functionality of the content must be operable through a keyboard interface without exception.
  • No Timing (SC 2.2.3): Timing is not an essential part of the event or activity presented by the content, except for real-time interactive events (like an auction).
  • Interruptions (SC 2.2.4): Users can postpone or suppress interruptions and non-emergency changes of content (e.g., pop-ups or notifications).
  • Re-authenticating (SC 2.2.5): If an authenticated session expires, the user can continue the activity without loss of data or re-entering data (except for re-entering a non-expired password).
  • Timeouts (SC 2.2.6): Users are informed about the duration of any user-controlled time limits and the required time is adjustable, or data is preserved after timeout.
  • Three Flashes (SC 2.3.2): No content flashes more than three times in any one-second period.
  • Animation from Interactions (SC 2.3.3): Motion animation triggered by user interaction can be disabled.
  • Location (SC 2.4.8): Information about the user's location within a set of web pages is available.
  • Link Purpose (Link Only) (SC 2.4.9): The purpose of each link can be determined from the link text alone.
  • Section Headings (SC 2.4.10): Sections within content have appropriate headings.
  • Focus Not Obscured (Enhanced) (SC 2.4.12): When a user interface component receives keyboard focus, the component and its focus indicator are fully visible and not hidden by other content.
  • Focus Appearance (SC 2.4.13): The focus indicator must meet specific size and contrast requirements to be clearly visible.
  • Target Size (SC 2.5.5): Target size for pointer inputs is at least 44 by 44 CSS pixels.
  • Concurrent Input Mechanisms (SC 2.5.6): Content does not restrict the user's choice of input modality (e.g., keyboard, mouse, touch, voice). 

Principle 3: Understandable

Unusual Words (SC 3.1.3): A mechanism is available to identify specific definitions for unusual words, including idioms and jargon.

  • Abbreviations (SC 3.1.4): A mechanism is available to identify the expanded form or meaning of abbreviations.
  • Reading Level (SC 3.1.5): When text requires reading ability more advanced than the lower secondary education level, supplemental content (e.g., summaries or illustrations) is provided.
  • Pronunciation (SC 3.1.6): A mechanism is provided to determine the correct pronunciation of words where the meaning is ambiguous without pronunciation.
  • Change on Request (SC 3.2.5): Changes of context occur only on user request or an alternative mechanism is available.
  • Help (SC 3.3.5): Context-sensitive help is available for complex tasks.
  • Error Prevention (All) (SC 3.3.6): For all forms, provide mechanisms for the user to reverse, check, and correct information before final submission.
  • Accessible Authentication (Enhanced) (SC 3.3.9): Authentication does not rely on a cognitive function test (like remembering a password or object recognition) unless the alternative is a non-cognitive test or there is a help mechanism.