Why do links become relative in the visual editor?
Why are internal hyperlinks transformed to relative when I save my text in Kupu?
Kupu converts absolute internal urls to relative to ensure consistent behaviour. Without the conversion links would be absolute when edited with Internet Explorer and relative when edited with Mozilla / Firefox.
When the HTML content needs to be used outside of Plone, e.g. in an e-mail, relative links should function correctly when you add a <base> tag in your pages.
Note that the links you see in Kupu's source view or "View source" of your browser may differ, since browsers may transform the content when you start editing in Kupu. Internet Explorer, for example, convert all links from relative to absolute.