HTML and CSS Reference
In-Depth Information
Optional Attributes
The i , b , u , and s elements don't have any optional attributes.
Superscript and Subscript: sup and sub
You may occasionally need to include superscript or subscript characters in your text, especially if you're
writing about mathematics or chemistry, or in certain languages that require it (French, for example). In
these cases, you can use the sup and sub elements, for superscript and subscript, respectively.
Superscript text is raised slightly higher than surrounding text, whereas subscripts are slightly lower.
Listing 4-38 shows an example of these elements: sup appears in the Pythagorean theorem for calculating
right triangles, and sub in the chemical formula for sulfuric acid.
Listing 4-38 . Examples of the sup and sub elements
<p>a <sup>2</sup> + b <sup>2</sup> = c <sup>2</sup> </p>
<p>H <sub>2</sub> SO <sub>4</sub> </p>
Figure 4-25 shows how a browser renders these elements. The contents of both elements appear slightly
smaller than the ordinary text around them.
Figure 4-25. The sup and sub elements
Whereas the sup and sub elements may seem presentational, when used appropriately they communicate
more meaning than a span would. A superscript numeral in a mathematical formula can signify an
exponent, so wrapping that numeral in a sup element would be preferable to styling it strictly with CSS; the
sup element itself carries that stylistic meaning. The element in that context says, “this is an exponent,” not
merely “this is raised text.” Exercise your own judgment and use these elements only when the content
warrants it.
Required Attributes
The sup and sub elements don't require any attributes.
Optional Attributes
There are no optional attributes for the sup or sub elements.
Text Annotations: ruby, rp, and rt
Ruby text is a small annotation that appears alongside other text, most commonly to act as a pronunciation
guide in logographic languages like Chinese, Japanese, and Korean. These languages may have
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