The citation that "Copy citation to clipboard" creates is in
Markdown format. It can be easily converted to an HTML link that will open the Calibre app and lead you straight to the exact text quoted. IMHO this is the ideal way to create citations for HTML-based books.
The citations might be different from those cited from a paper book and are not easily sharable with other people (unless you share the same library), but they are quite useful to the owner of the ebook.