--- a/docs/conf.py	
+++ a/docs/conf.py	
@@ -37,8 +37,12 @@ DIR = Path(__file__).parent.resolve()
 # ones.
 extensions = [
     "breathe",
-    "sphinxcontrib.rsvgconverter",
-    "sphinxcontrib.moderncmakedomain",
+    # Gentoo does not have packages for these extensions.  The docs appear
+    # to compile without them; it is not known if that skips part of the
+    # build, or if these are only used when generating documents other than
+    # the HTML that Gentoo uses.
+    #"sphinxcontrib.rsvgconverter",
+    #"sphinxcontrib.moderncmakedomain",
 ]
 
 breathe_projects = {"pybind11": ".build/doxygenxml/"}