Removing the autoloading of www.tmac for -Thtml runs also has a small back-compatibility concern: this autoloading has been documented since 2000's commit e092fba4, when the man page was introduced (as tmac/groff_markup.man, documenting the macro file then named tmac/tmac.arkup) in groff 1.16.  However, grohtml has also been characterized as alpha software since at least that time, so users should expect some behavioral churn.

I thought this was going to be easy...

(Also, I forgot and could not locate where this discussion with Dave took place--maybe he could remind me.)

diff --git a/ChangeLog b/ChangeLog
index 33e49ae28..984c72c2a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2022-08-20  G. Branden Robinson <g.branden.robinson@gmail.com>
+
+       * tmac/html.tmac: Stop automatically loading `www.tmac`.  That
+       auxiliary package is a convenience, not a necessity.
+       * NEWS: Add item.
+
+       Thanks to Dave Kemper for the discussion.
+
 2022-08-23  G. Branden Robinson <g.branden.robinson@gmail.com>

        * src/roff/troff/env.cpp (environment::set_family): Throw error
diff --git a/NEWS b/NEWS
index 43463ccee..0d92277a6 100644
--- a/NEWS
+++ b/NEWS
@@ -420,6 +420,17 @@ o On output devices using the Latin-1 character encoding ("groff -T
   sequences like `this' in the input (character remapping with 'char'
   requests and similar notwithstanding).

+grohtml
+------
+
+o The grohtml output driver no longer directs the formatter to
+  automatically load the "www.tmac" auxiliary macro package for all
+  documents rendered for the "html" and "xhtml" output devices; the
+  package is a convenience, not a necessity.  To use the macros
+  documented in groff_www(7), load the package with the "-m www"
+  command-line option to groff or use the ".mso www.tmac" request in
+  your document.
+
 gropdf
 ------

diff --git a/tmac/html.tmac b/tmac/html.tmac
index 887b2882c..fa2e0e165 100644
--- a/tmac/html.tmac
+++ b/tmac/html.tmac
@@ -533,8 +533,6 @@
 .defcolor yellow1 rgb #fffc17
 .defcolor yellowgreen rgb #52d017
 .
-.mso www.tmac
-.
 .ie '\[char97]'a' \
 .  mso latin1.tmac
 .el \

It's not.  Some macros that IMO properly belong in html.tmac are defined in www.tmac instead.

The comment before them even gives away this fact!

.\"
.\" supplementary macros used by other macro sets
.\"
.\" here are some tags specially for -Tps or -Thtml when invoked by
.\" pre-html to generate PNG images from postscript.
.
.\" --------------------------------------------------------------------
.\" HTML-DO-IMAGE - tell troff to issue an image marker which can be
.\"                 read back by pre-html
.\"
.de HTML-DO-IMAGE
.  if r ps4html \
.    nop \O[5\\$2\\$1.png]\O[1]\O[3]
.  if \\n[www-html] \
.    nop \O[5\\$2\\$1.png]\O[0]\O[3]
..
.
.\" --------------------------------------------------------------------
.\" HTML-IMAGE-END - terminate an image for HTML
.\"
.de HTML-IMAGE-END
.  if r ps4html \
.    nop \O[4]\O[2]\O[0]
.  if \\n[www-html] \
.    nop \O[4]\O[2]\O[1]
..
.
.nr www-png-no 0
.
.\" --------------------------------------------------------------------
.\" www-make-unique-name - generate another unique name in string
.\"                        'www-unique-name'
.\"
.de www-make-unique-name
.  nr www-png-no +1
.  ds www-unique-name \\*[www-image-template]\\n[www-png-no]\"
..
.
.\" --------------------------------------------------------------------
.\" HTML-IMAGE and friends tell grohtml that this region of text needs
.\"            to be rendered as an image.
.\"
.de HTML-IMAGE
.  \" generates a centered image
.  www-make-unique-name
.  HTML-DO-IMAGE \\*[www-unique-name] c
..
.
.de HTML-IMAGE-RIGHT
.  www-make-unique-name
.  HTML-DO-IMAGE \\*[www-unique-name] r
..
.
.de HTML-IMAGE-LEFT
.  www-make-unique-name
.  HTML-DO-IMAGE \\*[www-unique-name] l
..
.
.de HTML-IMAGE-INLINE
.  www-make-unique-name
.  HTML-DO-IMAGE \\*[www-unique-name] i
..
.
.\"  EQN-HTML-IMAGE and friends check to see whether the equation is
.\"                 not in an image, in which case it allows HTML
.\"                 (mathml) to be generated (if -Txhtml was specified).
.
.de EQN-HTML-IMAGE
.  \" generates a centered image
.  www-make-unique-name
.  EQN-HTML-DO-IMAGE \\*[www-unique-name] c
..
.
.de EQN-HTML-IMAGE-RIGHT
.  www-make-unique-name
.  EQN-HTML-DO-IMAGE \\*[www-unique-name] r
..
.
.de EQN-HTML-IMAGE-LEFT
.  www-make-unique-name
.  EQN-HTML-DO-IMAGE \\*[www-unique-name] l
..
.
.de EQN-HTML-IMAGE-INLINE
.  www-make-unique-name
.  EQN-HTML-DO-IMAGE \\*[www-unique-name] i
..
.
.\" --------------------------------------------------------------------
.\" EQN-HTML-DO-IMAGE - tell troff to issue an image marker which can be
.\"                     read back by pre-html
.\"
.de EQN-HTML-DO-IMAGE
.  ie r xhtml \{\
.    if !(\\n[.O] == 0) \{\
.      if r ps4html \
.        nop \O[5\\$2\\$1.png]\O[1]\O[3]
.      if \\n[www-html] \
.        nop \O[5\\$2\\$1.png]\O[0]\O[3]
.    \}
.  \}
.  el \
.    HTML-DO-IMAGE \\$*
..
.
.\" --------------------------------------------------------------------
.\" EQN-HTML-IMAGE-END - terminate an image for HTML
.\"
.de EQN-HTML-IMAGE-END
.  ie r xhtml \{\
.    if !(\\n[.O] == 0) \{\
.      if r ps4html \
.        nop \O[4]\O[2]\O[0]
.      if \\n[www-html] \
.        nop \O[4]\O[2]\O[1]
.    \}
.  \}
.  el \
.    HTML-IMAGE-END
..