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It's not clear what "run these substeps again" means. In particular, is it a subroutine call that comes back to the step once they've finished? (In which case for a string full of %s you'd eventually blow your stack.) I'm guessing you probably just want to jump back to substep 1 or something.
Ah yeah. Fixed: https://github.com/whatwg/url/commit/7d0df583746598ecf101ae678a72d8f3b0af0008