Problem:

The non-implementor-only view of the spec contains references to the term "space characters, but readers of the non-implementor view are not provided with a description of exactly which characters the spec defines as space characters

The cause is that the current source markup of the spec has class=impl on the entirety of the "Common parser idioms" subsection of the "Common microsyntaxes" section of the spec. And that subsection contains the dfn for "space characters".

Incidentally, the same subsection also contains the dfn's for "White_Space characters" and "alphanumeric ASCII characters".

Suggestion solution:

Remove the class=impl  on the div wrapping the "Common parser idioms" subsection, then put class=impl on only those following paragraphs in that subsection which are actually only relevant to implementors.

I'm not sure if some other non-implementor-only parts of the spec contain references to "White_Space characters", but if any do, I guess the dfn for that should be available in the non-implementor view as well.

And as far as "alphanumeric ASCII characters", it seems likely that some non-implementor-only part of the spec does make reference to that, so probably it too should not be class=impl