Cascading Style Sheets - HTML and XHTML: The Definitive Guide - page 436

roman value for the list-style property of the <li> tag. When it sees
an <li> tag nested within two <ol> tags, the browser uses the upper-
alpha list style. Nest an <li> tag within three and four <ol> tags, and
you'll see the decimal and lower-alpha list styles, respectively. Compare
Figure 8-1 , displayed from the preceding example, with using the <ol>
tag's type attribute to achieve similar effects, as shown in Figure 7-7 in
Chapter 7 .
Figure 8-1. Nested ordered list styles
Similarly, you may impose a specific style on tags related only by con-
text. For instance, this contextual style definition colors the emphasis
( <em> ) tag's contents red only when it appears inside a level-1 header
tag ( <h1> ), not elsewhere in the document:
h1 em {color: red}