%x-elements;
X - Generated Text/Punctuation Elements
Elements that can be included along with data characters inside the content model of the <x> element.
Model
<!ENTITY % x-elements "| %emphasis.class; | %phrase-content.class; | %subsup.class; | %x.class;" >
Expanded Model
| bold | fixed-case | italic | monospace | overline | overline-start | overline-end | roman | sans-serif | sc | strike | underline | underline-start | underline-end | ruby | named-content | styled-content | sub | sup | x