A quick note that as of this post, section 5.1 starts:
Selectors Level 3 [SELECT], is nearing specification completion and already has seen substantial adoption, following past development of Levels 1 and 2. There may eventually be Levels 4 and beyond, which will require more sophisticated processors.
That was true when Section 5.1 was written, but obviously it’s out of date now. I propose that we just drop those sentences, since the ones that follow are the real meat of the section and they aren’t bound any particular point in time.