Readability statistics, such as those generated by some word processors may be useful indicators of clarity and simplicity.
Better still, ask an experienced (human) editor to review written content for clarity.
It is a stable document and may be used as reference material or cited as a normative reference from another documents.
W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment.
These guidelines do not discourage content developers from using images, video, etc., but rather explain how to make multimedia content more accessible to a wide audience.
This is a reference document for accessibility principles and design ideas.
In particular, new features of HTML 4.0 or CSS 1 or CSS 2 may not be supported.