Oh, just remembered from calligra discussions when in a "relaxing situation": svg:x is optional,. and defaults to 0 if not given. So if x is undefined it should be treated as 0 here and the rule still be calculated if parentRect && pageRect are given.
Same for y.
Also quite fragile, as this relies on first time rendering, where no rules are yet given for the frame (otherwise getBoundingClientRect() would not have useful values). Hopefully good enough for now.