The First-child Pseudo-class selects an element context that is the
first child node of another selector in the document element tree. This
meaning may be clear, but the targets (which is the child element, and
which is the parent) may be confusing. If a selector looks like this:
"strong:first-child", it means "all STRONG elements
that are the first-child node of other elements, NOT "all
first-child nodes of STRONG elements." If you want to specify all
first-child nodes of STRONG elements, the syntax would look like:
"strong :first-child" (notice the space in there
indicating a descendent selector.)
Example
Ext/Doc: p a:first-child
{ color: green }
(selects the first hyperlink child element of any P elements and turns the content green.)