Stop stripping of / from self closing tags

[evilsupernasty]

I am working on a project that requires XHTML. How can I prevent self-closing tags from having their closing slash removed? I have disabled optimizers for *.html files in .parcelrc, but that makes no difference. I have also looked at the options for htmlnano/posthtml but there does not seem to be an option related to closing slashes. Anyone have any ideas?