POUR - WCAG

The Web Content Accessibility Guidelines (WCAG), developed by the World Wide Web Consortium, are technical standards that help make the digital world accessible to people with disabilities. WCAG standards are rooted in four main principles: perceivable, operable, understandable, and robust, often called POUR.

Perceivable

Information must be perceivable to people using only one of their senses, so they understand all related content.

My thots: Check the contrast outside under bright lighting conditions. Ensure the audio understandable in a noisy room. Both light and dark backgrounds are common, ensure text color adjusts. There are various forms of color-blindness, check using a color-blind simulator and/or in grayscale to ensure no meaningful information is lost--that is, a red circle and a green circle are the same when the color is muted. Wear hearing protection to ensure alternatives to increasing the volume are available when possible. Blur your vision (or remove your glasses) to ensure the text and image size can be increased to improve readability. Ensure temporary notifications display sufficiently long to be recognized when distracted. Display on a large projected image (where color and contrast may vary).

Operable

End users must be able to interact with all webpage elements. For instance, your website should be easily navigable with just a keyboard or voice controls for non-mouse users.

My thots: Wear gloves, thus causing a loss of precise touch. Use just a keyboard--don't rely on a mouse cursor. Use just a touch-screen--don't rely on hover-text. Use a small screen (e.g., mobile phone). Use a connected display some distance away with just a mouse, for example, use a wide-screen TV and a mouse on an uneven surface or a TV remote.

Understandable

The principle is just what it seems—end users must be able to understand web page content and functionality information.

My thots: this principle should be true for all means of communication, but it is very difficult especially given the diversity of the audience. Those who don't know English are left with machine translation. As a person with an analytical, engineering mind, I find that most web sites do a very poor job with understandability for me because their audience is primarily non-technical, emotionally driven, neurotypical people. If I don't already know what a company does, visiting their website seldom gives me more than just a clue about what they do. For example, take this headline, "Essential Software - For the People Who Change the World." OK, at least I know it's a software company, but what kind and is it cloud-based, mobile, desktop, a framework to be integrated, self-service or requires technical professional consultants to configure? The next subheading helps: "Software built for fundraising, nonprofit accounting, education, CSR and more." Now, I know what markets this software is design to meet. There's a financial component and an educational component and CSR, whatever that is--though if I don't know what CSR means, this software is probably not what I need. But then the subheading continues with "Fueling bigger, bolder impact — everywhere." What does that mean? 

Robust

Your website must effectively communicate information to all users, including users of assistive technologies, and remain compatible with evolving technologies and user needs.

My thots: let's face, this is a goal seldom reached because of expense and lack of knowledge of capabilities. Also, not everything needs to be accessible, at least not on a practical level. For example, a website to sell a product that requires sight to use has little need for a website compatible with those who can't see. Technology has advanced making some best practices no longer important. For example, using relative styles (i.e., not "pt") for font size is not nearly as important now that browsers allow the magnification of the entire screen to scale. 

My biggest fight has been with web page designers who want to control the appearance of the page and not allow the viewer to decide how they want to interact with the content.

source: https://www.wcag.com/resource/what-is-wcag/