Recognizing Semantics - HTML5 Mastery: Semantics, Standards, and Styling - page 123

The link targets the earlier dfn element using a fragment identifier, so the user can
jump back to where the meaning of “NFA” was defined in the text (in the parentheses
that followed dfn ) ( Figure 3-10 ).
Figure 3-10. Abbreviations can be linked back to the first instance of where they were defined
using the dfn element.
Subscript and superscript: sub and sup
There is a joke in science circles where two people are in a restaurant and the waitress
asks what they would like to drink. The first replies “H 2 O.” The waitress then looks at
the second patron, who thinks for a moment and says, “I'll have H 2 O too!”
A short while later the waitress returns with their drinks. The first person takes a gulp
from theirs. The second person gulps from theirs and falls to floor foaming at the mouth.
“Idiot,” the first exclaims.
The joke is that the second person—instead of appearing brainy like the
first—inadvertently ordered “H 2 O 2 ” (hydrogen peroxide) instead of H 2 O (water).
While sup and sub (superscript and subscript) appear presentational in nature, as
this joke demonstrates, they convey important meaning about the text they surround.
Consider the two following equations:
• e=mc 2
• e=mc2
Although they look alike, only one of the preceding equations is Einstein's; spelled
out, the former equation is “e equals m times c squared,” while the latter is “e equals m
times c times 2.”
Note For equations more complex than this, you should use Mathematical Markup
Language (MathML), which is defined in a separate specification from that which de-
scribes HTML but may be embedded in an HTML document.
Or how about this:
• H 2 O
• H2O