Self-closing markup tags should be disallowed or handled correctly

[mbollmann]

The XML has a self-closing <b/> tag, which is technically allowed by the schema. I believe it’s not valid HTML, though, which may be why some part of the Hugo pipeline turns it into a <b>. I guess we could (i) check if the schema can be written to disallow empty markup elements, and/or (ii) replace these with something valid in our MarkupText.as_html().

Originally posted by @mbollmann in #6407

[mbollmann]

Not sure if there’s a way to disallow empty markup tags, as regex patterns or minimum lengths can only be applied to xsd:string, not text, but for interleaving plain text with tags we need to use text IIRC.

cf. https://stackoverflow.com/questions/39624122/how-to-require-mixed-element-to-be-non-empty-in-relaxng

[nschneid]

What about a regex test to flag <[biu]\s*/>?

[mbollmann]

A regex where? Regexes can't be added to "text" elements in RelaxNG AFAIK.

[nschneid]

No I mean add an ad hoc test somewhere in the pipeline.