2.1.3 [Ruby:2001] Section 2.7, The rp element
The rp element can be used in the case of simple ruby markup to specify characters that can denote the beginning and end of ruby text when user agents do not have other ways to present ruby text distinctively from the base text. Parentheses (or similar characters) can provide an acceptable fallback. In this situation, ruby text will only degrade to be rendered inline and enclosed in the fallback parentheses. This is the least inappropriate rendering under the condition that only inline rendering is available. The rp element cannot be used with complex ruby markup.
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The rp element is allowed within complex ruby elements.