BTW: Loading this tiny test case in Safari and Firefox shows both browser engines support background color on a span.
Code:
<html>
<head>
<title>huh</title>
<style>span.highlight { background: #ccc };</style>
</head>
<body>
<p>By clearly defining the purpose and goals of your business and how you want people to use your site, you will be more equipped to respond to design and development suggestions. <span class="highlight">Every issue that comes up</span> in the development process can then be evaluated against the primary goals of your marketing.</p>
</body>
</html>
So I do not understand why a Firefox based Epub reader would not.