Feed-back from the gnome people is:
There weren’t any major features added [to libxml2] in the last ~10 years and libxml2 is only barely maintained. Have a look at Gumbo if you need a HTML5 parser written in C with bindings for many languages like Ruby.
and a workaround would be to use HTML4 in the preview window as per this answer (dunno if the browser fix was already present back then).
Just a last remark: As you’re basing your project on a project that’s basing their code on a 10 year old barely maintained library, it might be useful to have a look at something else and end-users will have to live with doing the strike-through manually over multiple lines until then…
¯\_(ツ)_/¯