The Web Content Accessibility Guidelines (WCAG) is an initiative run by World Wide Web Consortium (W3C) to ensure web content is universally accessible. For more details on WCAG see this document. 

WCAG is organised around four main principles, often referred to by the acronym POUR: Perceivable, Operable, Understandable, and Robust. These principles allow web content providers (which includes market research agencies and their online surveys) to provide digital content that is accessible to users regardless of their abilities or the assistive technologies they use.

At a high-level the four principles can be described as follows:

The original accessibility guidelines focused on people with visual impairment.  Websites were simpler and so they mostly provided the opportunity to consume written information.  Perceivable means that text content must be visually clear and non-text content should include alternative textual descriptions. For example, all images should include a description of the image.

Read this article for more details.

In summary: Survey content will be legible, regardless of how it is accessed.

As websites became more dynamic, it was important to ensure they could still be used by the disabled. At the same time, the definition of accessibility extended to those with motor-based disabilities (for example older people with hand tremors).  Operable states that content should be easy to discover and navigate.

Read this article for more details.

In summary: Survey functionality will be available, regardless of how it is accessed.

Accessibility has, in the last few years, extended to cover cognitive disabilities such as Asperger’s syndrome, obsessive compulsive disorder and dyslexia. This has resulted in new guidance around reading age and complexity of language. The expectation is that websites should operate predictably and should avoid complex language unless absolutely necessary.

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In summary: Survey content will make sense, regardless of how it is accessed.

In addition to guidelines around device agnostic websites and mobile first, Robust asks that the content can be expressed via a wide range of devices and browsers.  Screen readers and braille readers are two types of output that rely on well-structured, clear content to make the online content meaningful.

Read this article for more details.

In summary: Survey content and functionality will still work, regardless of how it is accessed.

Each of these principles is explored further in four separate articles.