relative links, how?

The real question is why would you want to use relative paths. There are all sorts of reasons not to, which is why WordPress doesn’t use them. See, for instance,

1. https://core.trac.ww.wp.xz.cn/ticket/17048#comment:2
2. http://lists.automattic.com/pipermail/wp-hackers/2010-November/036195.html
3. https://yoast.com/dev-blog/relative-urls-issues/

If we knew why you wanted to use relative URLs, we could probably provide a more appropriate solution, which will not cause things to break further down the road.