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 Indicator (URI)
<inline-supplementary-material> Inline Supplementary Material
<related-article> Related Article Information
<related-object> Related Object Information
<hr> Horizontal Rule
<bold> Bold
<italic> Italic
<monospace> Monospace Text (Typewriter Text)
<overline> Overline
<overline-start> Overline Start
<overline-end> Overline End
<roman> Roman
<sans-serif> Sans Serif
<sc> Small Caps
<strike> Strike Through
<underline> Underline
<underline-start> Underline Start
<underline-end> Underline End
<alternatives> Alternatives For Processing
<inline-graphic> Graphic, Inline
<private-char> Private Character (Custom or Unicode™)
<chem-struct> Chemical Structure (Display)
<inline-formula> Formula, Inline
<tex-math> TeX Math Equation
<mml:math> Math (MathML 2.0 Tag Set)
<abbrev> Abbreviation or Acronym
<milestone-end> Milestone End
<milestone-start> Milestone Start
<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
<x> X - Generated Text and Punctuation