Sara Soueidan, who else, has written an extensive article on how to mimic relative positioning in an SVG. The gist: nest your SVGs.
Whilst her examples are – as per usual – very extensive and complete, I took the liberty on simplifying her final example:
In the simplified example
#bird is positioned at
x=50%,y=50% of it’s parent
#birdwrapper, which itself is positioned at
x=698,y=1219 in it’s own enclosing parent, mimicking absolute-relative positioning wich we know from CSS. As an extra visual indicator both
#birdwrapper contain an extra
<rect> to visualize their bounding boxes.