Selecting an element by its content using pure CSS isn't possible as far as I know.
You can do some other things:
1. Add a class
<div>
<dl class="abc">
<dt class="a">A</dt>
<dd style="display: none;"></dd> <dt>B</dt>
<dd style="display: none;"></dd>
</dl>
</div>
So you can use the following selector:
.a {
/* CSS code here */
}
2. Add an id
<div>
<dl class="abc">
<dt id="a">A</dt>
<dd style="display: none;"></dd> <dt>B</dt>
<dd style="display: none;"></dd>
</dl>
</div>
So you can use the following selector:
#id {
/* CSS code here */
}
You can only use one ID per page, so this would be best if you want to be sure only this element is selected.
3. Select by DOM structure
<div>
<dl class="abc">
<dt>A</dt>
<dd style="display: none;"></dd> <dt>B</dt>
<dd style="display: none;"></dd>
</dl>
</div>
Selector:
div > dl > dt {
/* CSS code here */
}
These would all work, you should pick which one suits your needs.