Copy text from whatwg/sg#184 (comment)

ruby/ruby-agreement.md

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+# Agreement on HTML Ruby Markup
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+We understand that the W3C will specify the extended HTML Ruby markup described in [PR#6478](https://github.com/whatwg/html/pull/6478) as a REC track document. We believe this is within the bounds of our MoU with the W3C, and while it is not the ideal outcome (having Ruby documented in two places), we understand that it serves the goals for the W3C's constituencies and existing implementations to have an easily referencable document as well as honoring the strict requirements of the WHATWG that the living standard not contain new additions that don't have multi-browser-engine implementer commitments.
+
+We understand that the SOTD section of the W3C Ruby Extension spec will state our joint intention that the extended ruby markup definitions be fully integrated into the WHATWG HTML specification once they [meet WHATWG's inclusion criteria](https://whatwg.org/faq#adding-new-features) under section 10.2 of the 2019 MOU.
+
+We understand that W3C editors will offer pull requests to keep the HTML spec in sync, both technically and editorially, for the subset of features defined in both specs (goal is to reduce the deltas such that the only differences will be the addition of the two new elements `<rb>` and `<rtc>`); we expect WHATWG HTML editors to work with the W3C Ruby Extension spec authors to merge those pull requests in support of that goal.