This is everything to do with making web content more accessible, hence: Web Content Accessibility Guidelines.
These guidelines are a collection of recommendations, that aren’t technology related, and make web content more accessible for people with a disability, such as the blind (or visually impaired), or those with cognitive or neurological disabilities etc.
The WCAG was developed by the W3C (World Wide Web Consortium).
All of the Flemish government’s websites and applications must be as accessible as possible. On 5 July 2018, the Belgian Chamber of Representatives approved a bill on the accessibility of the websites and mobile applications of public authorities. (You can read a detailed article about the legal framework here)
You can find the full detailed WCAG checklist here, divided into the 4 aforementioned principles. The more recommendations you can follow, the more accessible your web content will be.