Both of the following codes seem to work properly to style the span element:

Code:
<style>
    div p span {
        font: 18px arial;
        color: red;
    }
</style>
<div>
    <p>
        <span>
Hello, world!
</span>
    </p>
</div>
Code:
<style>
    div span {
        font: 18px arial;
        color: red;
    }
</style>
<div>
    <p>
        <span>
Hello, world!
</span>
    </p>
</div>
But I'm not sure if the second one is the right coding and if there's a difference between them, for example regarding browser support.