Thank you, that's a good lead, but actually, my process is the other-way around
I have an HTML input (which I can't share for now for it's generated on the fly as a temporary file right before passing it down to ebook-convert; I need to find a way to snapshot it before it gets removed from the hard-drive) and that HTML is converted into a PDF thanks to ebook-convert.
I have access to the raw HTML templates eventually combined into one fully-fledged document fed to ebook-convert, though, so that might help. Within those templates, I have no ".page" tag, yet that CSS selector is being recognized by the convert tool eventually, for I can apply styles to that classname and they get properly rendered in the generated PDF document. I can't get styles assigned to the CSS selector "body" to be rendered, though. That's why I'm a bit confused about the final structure used by ebook-convert: it doesn't 100% match the HTML templates. Any insights?