Used to provide spacing and a parenthesis to surround Ruby Text (<rt>), for older systems that do not handle Ruby formatting properly
This optional element never holds any text except a space, an open parenthesis, or a close parenthesis:
<ruby><rb>text></rb> <rp>(</rp> <rt>annotation</rt><rp>) </rp>
<!ELEMENT rp (#PCDATA) >
Text, numbers, or special characters