2.1.3 [W3C-RUBY] Section 2.7, The rp element


The specification states:

 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 

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The rp element is allowed within complex ruby elements.