[cssom-1] Indicating when a style sheet is script-blocking is duplicated several times

In the HTML Standard's Interactions of styling and scripting section we have the definition of the "style sheet ready" flag for CSS style sheets. We also have a list of conditions which, if met, make a style sheet a script-blocking one. Not too long ago this section was reformatted, and another condition has been added.

The "style sheet ready" flag is seemingly never used or set, and we're going to change (and maybe rename?) this to be a flag that makes a style sheet that was previously script-blocking, no longer script-blocking. When searching around for usages of the flag, I noticed that some text at the bottom of both the xml-stylesheet process instruction and Link header requirements sections mention the conditions that make a style sheet a script-blocking one. This seems like normative information that is duplicated from the HTML Standard, and can easily get out of sync (like with the adding of more conditions as we've seen).

Would the editors accept a PR from me that replaces these paragraphs with something like a "Note" linking to the appropriate section in the HTML Standard so this logic might stay in one place?

/cc @emilio @annevk

Start with the CSSOM sections on the xml-stylesheet processing instruction and Link header requirements, then compare their script-blocking conditions with the HTML Standard's Interactions of styling and scripting section. Replace the duplicated normative paragraphs with a note linking to the HTML definition, so the conditions are maintained in one place.