WCAG’s POUR Principles are:

Perceivable

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

When users understand what's happening in an application and on their screens, they can decide how to interact with the content properly. The perceivable principle is all about making sure our users know what is going on within the application.

As developers/Designers, we must consider constraints such as:

• Sight
• Hearing
• Touch 
• Transformability – Consider appearance on different devices and in differing contexts
• Presentation Layer – Elements, controls, and components presented in a way that the users understand intuitively
• Content vs. Style & Presentation – If the presentation layer goes away, is the content still readable/logical?

Operable

WCAG Definition: User interface components and navigation must be operable.

If users can perceive what’s happening, they’ll want to take some kind of action. The operable principle is all about making sure that people can actually use the application using different tools.

• Input methods – How will this work if users only use a keyboard, instead of a mouse? 
• Navigation – How clear and easy is the navigation of a site or a specific control? Can the user skip the repetitive navigation elements? Can the user properly scan the content of a specific screen using a screen reader function? 
• Controlling timing, time limits, and seizure prevention – Are we providing users adequate time to complete a task? Provide users control to stop animations.

Understandable

WCAG Definition: Information and the operation of the user interface must be understandable.

Applications should be easy to understand: this refers to both the content and interactions.

• Meaningful language – The verbiage should be simple/clear for all users to understand. If technical terms are used, a definition of the term should be provided.
• Predictability – Can a user predict the interactions or behaviors based on clear visual metaphors, and/or based on the learned behaviors within the applications? Maintain consistent patterns in the UI.
• Identifying and overcoming errors – Assist the user in avoiding and overcoming errors efficiently. Strive to prevent errors by providing clear instructions and visual cues.
• Help – Provide adequate help content, so that users can avoid failure or errors. Provide helpful messaging and easy access to error resolution/mitigation steps.

Robust

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

Strive to develop according to standards that include well-formed and semantically valid code to ensure we meet the final principle of robustness. This includes adapting to future technologies accordingly.

Remember to consider the senses, modes of human-to-computer interaction, contexts of use, and above all, to empathize with your users

WCAG Quick Reference