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.