Do not add HTML mixin if not building html #539

Job	Run time
a11y-tests (ubuntu-latest, 3.12)	3m 25s
build PST docs (ubuntu-latest, 3.12)	1m 19s
run-pytest (ubuntu-latest, 3.9)	1m 30s
build PST docs (macos-latest, 3.12)	2m 29s
build PST docs (windows-latest, 3.12)	4m 49s
run-pytest (ubuntu-latest, 3.10)	1m 43s
build PST docs (ubuntu-latest, 3.9, 6.1)	1m 20s
run-pytest (ubuntu-latest, 3.11)	1m 23s
run-pytest (ubuntu-latest, 3.12)	1m 49s
run-pytest (ubuntu-24.04, 3.11)	1m 43s
run-pytest (ubuntu-24.04, 3.12)	2m 49s
run-pytest (macos-14, 3.10)	2m 39s
run-pytest (macos-14, 3.11)	2m 18s
run-pytest (macos-14, 3.12)	2m 23s
run-pytest (windows-latest, 3.9)	2m 51s
run-pytest (windows-latest, 3.10)	3m 55s
run-pytest (windows-latest, 3.11)	3m 41s
run-pytest (windows-latest, 3.12)	3m 0s
run-pytest (ubuntu-latest, 3.9, 6.1)	1m 39s
run-pytest (ubuntu-latest, 3.12, dev)	2m 2s
Check coverage	49s
lighthouse-audit	4m 11s
profiling	30s
	54m 17s

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