POUR Principles

Every success criterion in WCAG is organized under one of four principles: Perceivable, Operable, Understandable, and Robust. These principles (often shortened to POUR) have remained the foundation of the standard since WCAG 2.0 in 2008. Understanding them makes the rest of the guidelines much easier to navigate.

Perceivable

Information and user interface components must be presentable to users in ways they can perceive.

This principle addresses the most basic requirement: if a user cannot perceive the content, nothing else matters. Common requirements include:

• Text alternatives for non-text content (images, icons, charts)
• Captions and transcripts for audio and video
• Content that can be presented in different ways without losing meaning (for example, through semantic structure)
• Sufficient color contrast and the ability to resize text

Practical example: An image of a product must have meaningful alternative text. A video must have accurate captions. Text and interactive elements must meet minimum contrast ratios so they remain readable.

Robust

Content must be robust enough that it can be interpreted reliably by a wide variety of user agents, including assistive technologies.

This principle is largely about clean, standards-compliant code. The main requirements are:

• Valid, well-formed markup
• Proper use of names, roles, and values so assistive technologies can understand components
• Compatibility with current and future user agents

Practical example: Custom controls (such as a custom dropdown or modal) must expose the correct ARIA roles, states, and properties so screen readers can announce them accurately. Invalid or incomplete markup often breaks this communication.

How the Principles Work Together

The four principles are not isolated checklists. A single interface problem can violate more than one principle at the same time. For example, a button that cannot be reached by keyboard fails Operable; if it also lacks a proper accessible name, it fails Robust and may also fail Perceivable.

When you evaluate a page, it is useful to ask four simple questions:

1. Can users perceive the information?
2. Can they operate the controls?
3. Can they understand the content and the interface?
4. Will assistive technologies be able to interpret it reliably?

If the answer to any of these is “no,” you have found a barrier that needs attention.

Key Takeaway

POUR is the organizing framework of WCAG. Once you are comfortable with the four principles, the individual success criteria become much easier to understand and apply. In the next post we look at those success criteria in more detail — what they are, how the conformance levels work, and how to read them effectively.

Next in the series: WCAG Success Criteria