Fix up HTML output to clean up browser warnings #4104

Job	Run time
copyright_headers	17s
clang-format	15s
black	26s
cppcheck	5m 42s
flake8	19s
forbidden_types	11s
lychee_IGNORED	24s
mypy	33s
pydocstyle	20s
pylint	43s
rstcheck	27s
unused_names	18s
vale	50s
build_linux (ubuntu-20.04, gcc, boost, optimize, warning)	15m 25s
build_linux (ubuntu-20.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	24m 25s
build_linux (ubuntu-20.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	19m 47s
build_linux (ubuntu-20.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	38m 22s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	26m 27s
sphinx-rtd	3m 53s
sphinx-conda	0s
	2h 19m 4s

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