Looking at the manual today, regarding the "interword space" section, fontspec's WordSpace and \spaceskip seem like completely different features to me. WordSpace only applies when that font is in use, right? \spaceskip does not.
Regarding the "post-punctuation space" section, it is still conventional/standard in the US to use non-frenchspacing, not "old-fashioned". (That's why it is the default in TeX, after all.) Sorry to lose the pun.
Diff appended for your consideration. If work on fontspec ever resumes ... --thanks, karl.
--- fontspec-doc-featset.tex 2020-01-23 13:54:34.000000000 -0800
+++ /tmp/new 2021-01-30 09:36:53.305096962 -0800
@@ -419,6 +419,5 @@
-While the space between words can be varied on an individual
-basis with the \TeX\ primitive \cmd\spaceskip\ command, it is
-more convenient to specify this information when the font is
-first defined.
+While the space between words can be varied with the \TeX\ primitive
+\cmd\spaceskip\ command, \pkg{fontspec} also supports changing the
+interword spacing when a given font is loaded.
@@ -458,4 +457,4 @@
space after some punctuation in its goal of justifying the lines of text.
-Generally, this is considered old-fashioned, but occasionally in small amounts the
-effect can be justified, pardon the pun.
+This is conventional for US typesetting, though not in most of the rest
+of the world.
Looking at the manual today, regarding the "interword space" section, fontspec's WordSpace and \spaceskip seem like completely different features to me. WordSpace only applies when that font is in use, right? \spaceskip does not.
Regarding the "post-punctuation space" section, it is still conventional/standard in the US to use non-frenchspacing, not "old-fashioned". (That's why it is the default in TeX, after all.) Sorry to lose the pun.
Diff appended for your consideration. If work on fontspec ever resumes ... --thanks, karl.