The sup tag displays text as a superscript, and the sub tag displays it as subscript. The exact position and size of super/sub-scripted text is defined by the font. The two elements are equivalent to <span font-size="subscript" vertical-align="super"> and <span font-size="subscript" vertical-align="sub"> respectively.
<p>Einstein insists that E=MC<sup>2</sup></p> <p>Remember that H<sub>2</sub>O is safe to drink, while H<sub>2</sub>SO<sub>4</sub> is not</p>