many free static site hostings don't support XML index files so you HAVE to use the HTML output mode if you want to support neocities, gitlab pages, …
but as far as xsltproc is concerned it clearly needs a lot of love
look once you learn XSLT it's simply the easiest SSG
i figured this out btw xsltproc just doesn't know that HTML elements have a namespace in 2022; you have to null-namespace them
somebody smart with free time on their hands please fix xsltproc to ensure that it never outputs closing tags for void elements in the HTML output mode because i’m presently getting them and it’s annoying