Spec contains RFC 2119 requirements for authors

[tidoust]

The specification contains statements that set requirements on authors using RFC 2119 terms:

• "the author SHOULD populate all the mip levels"
• "The author SHOULD redraw the content of the layer at the next XR animation frame"
• "The author SHOULD fill in the RGB components of this texture..."

The use of RFC 2119 is confusing because authors are not a valid class of products for a specification that is to be implemented by user agents (said differently, authors do not implement W3C specs, the test suite that checks conformance to the specification will not contain tests for these statements). It is certainly useful to include guidelines for authors, but these guidelines can only be informational in nature.