Heading over the first column (the term column) of a definition list (two-part list).
Any combination of:
Text, numbers, or special characters
<email> Email Address
<ext-link> External Link
<uri> Uniform Resource Identifier (URI)
<bold> Bold
<italic> Italic
<monospace> Monospace Text (Typewriter Text)
<overline> Overline
<roman> Roman
<sans-serif> Sans Serif
<sc> Small Caps
<strike> Strike Through
<underline> Underline
<alternatives> Alternatives For Processing
<inline-graphic> Graphic, Inline
<chem-struct> Chemical Structure (Display)
<inline-formula> Formula, Inline
<abbrev> Abbreviation or Acronym
<named-content> Named Special (Subject) Content
<styled-content> Styled Special (Subject) Content
<fn> Footnote
<target> Target of an Internal Link
<xref> X (cross) Reference
<sub> Subscript
<sup> Superscript