It is valid HTML, but not valid XHTML. Since ePUB must contain XHTML, it will not work. You should use the following:
Code:
.strike {text-decoration: line-through;}
in the stylesheet and in the XHTML:
Code:
<p>I’d just pull out my handy <span class="strike">abacus , slide rule</span> , calculator to do my figuring. </p>