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he Step 13 of [1] (13. If root has a ruby element ancestor, then abort these steps.) prohibits the the processing of nested ruby. However, nested ruby feature is already implemented in Chrome and Safari, and it is likely that someone in the world may already use it. Also, it is unclear that what would be the expected result of "aborting" the process. As of it, I think that Step 13 of [1] may be no longer necessary, and would like to propose to remove the step 13 from the specification. [1] ... http://www.w3.org/html/wg/drafts/html/CR/text-level-semantics.html#segmentation-and-categorisation-of-ruby
I agree with your assessment, and have therefore removed this limitation.