diff --git a/docs/Makefile b/docs/Makefile
index 8c2f78b..3e22b60 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -1,71 +1,71 @@
-SPHINXOPTS =
+SPHINXOPTS ?=
 SPHINXBUILD = python3 -c 'import sphinx, sys; sys.exit(sphinx.main(sys.argv))'
 SOURCEDIR = .
 BUILDDIR = _build
 HTMLDIR = $(BUILDDIR)/html
 SWHPKGDIR = `python3 -c 'import swh; print(swh.__path__[0])'`
 
 INSTALL_HOST = pergamon.internal.softwareheritage.org
 INSTALL_DIR = /srv/softwareheritage/docs/webroot/devel
 INSTALL_GROUP = swhdev
 INSTALL_PERMS = g+rwX
 
 SPHINXAPIDOC = sphinx-apidoc
 APIDOC_DIR = apidoc
 APIDOC_OPTS = --ext-viewcode
 APIDOC_EXCLUDES = */tests */tests/* */*/tests/* */*/*/tests/*
 APIDOC_EXCLUDES += */migrations */migrations/* */*/migrations/* */*/*/migrations/*
 APIDOC_SWH_EXCLUDES = $(patsubst %,$(SWHPKGDIR)/%,$(APIDOC_EXCLUDES))
 
 apidoc_dep = apidoc-stamp
 
 html: sphinx/html
 
 sphinx/html: links-stamp apidoc-stamp images-stamp rec-build-stamp
 
 links-stamp:
 	bin/ln-sphinx-subprojects
 	touch $@
 
 apidoc: $(apidoc_dep)
 apidoc-stamp:
 	$(SPHINXAPIDOC) $(APIDOC_OPTS) -o $(APIDOC_DIR) $(SWHPKGDIR) $(APIDOC_SWH_EXCLUDES)
 	touch $@
 
 images-stamp:
 	$(MAKE) -C images
 	touch $@
 
 # Build documentation recursively in all packages, to make sure that auxiliary,
 # non-sphinx managed documentation artifacts (e.g., schema diagrams) are also
 # built.
 rec-build-stamp: $(wildcard ../../swh-*/docs/*.rst)
 	$(MAKE) -C ../../ docs-assets
 	touch $@
 
 clean: sphinx/clean
 	bin/ln-sphinx-subprojects --remove
 	$(MAKE) -C images clean
 	rm -f *-stamp
 	rm -f $(APIDOC_DIR)/*
 
 distclean: clean
 	make -C ../../ docs-clean
 
 help: sphinx/help
 
 sphinx/clean:
 	@$(SPHINXBUILD) -M clean "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS)
 
 sphinx/%:
 	@$(SPHINXBUILD) -b $* "$(SOURCEDIR)" "$(BUILDDIR)/$*" $(SPHINXOPTS)
 
 install: html
 	test -d $(HTMLDIR)
 	rsync -rlvuz --delete $(BUILDDIR)/html/ $(INSTALL_HOST):$(INSTALL_DIR)/
 	ssh $(INSTALL_HOST) \
 		"find $(INSTALL_DIR) -not -group $(INSTALL_GROUP) -exec chgrp -v $(INSTALL_GROUP) {} + ; \
 		 find $(INSTALL_DIR) -not -perm -ug=rw,o=r -exec chmod -v ug+rw,o+r {} + ; \
 		 find $(INSTALL_DIR) -type d -not -perm -g=xs,ou=x -exec chmod -v g+xs,ou+x {} +"
 
 .PHONY: help html clean distclean install
diff --git a/tox.ini b/tox.ini
index 8fe8833..31b1555 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,28 +1,29 @@
 [tox]
 envlist=flake8
 
 [testenv]
 basepython = python3
+passenv = SPHINXOPTS
 
 [testenv:sphinx]
 deps =
   django < 2
   .[testing]
   pifpaf
 commands =
   {envpython} -m pifpaf run postgresql -- make -C docs html
 
 [testenv:sphinx-dev]
 deps =
   django < 2
   -rrequirements-swh-dev.txt
   pifpaf
 commands =
   {envpython} -m pifpaf run postgresql -- make -C docs html
 
 [testenv:flake8]
 skip_install = true
 deps =
   flake8
 commands =
   {envpython} -m flake8