How can I count the number of elements that match my CSS selector?

I am trying to use SeleniumRC to test my GWT App and am trying to match elements using CSS selectors.

I want to count the number of enabled buttons in the following HTML.

A button is enabled if it is under a <td> with class="x-panel-btn-td " and disabled if it is under a <td> with class="x-panel-btn-td x-hide-offsets".

So basically, I want to retrieve the number of buttons under all <td>s with the class x-panel-btn-td.

<table cellspacing="0">
    <tbody>
    <tr>
        <td id="ext-gen3504" class="x-panel-btn-td ">
            <em unselectable="on">
                <button id="ext-gen3506" class="x-btn-text" type="button">OK</button>
            </em>
        </td>
        <td id="ext-gen3512" class="x-panel-btn-td x-hide-offsets">
            <em unselectable="on">
                <button id="ext-gen3506" class="x-btn-text" type="button">Yes</button>
            </em>
        </td>
        <td id="ext-gen3520" class="x-panel-btn-td">
            <em unselectable="on">
                <button id="ext-gen3506" class="x-btn-text" type="button">No</button>
            </em>
        </td>
        <td id="ext-gen3528" class="x-panel-btn-td x-hide-offsets">
            <em unselectable="on">
                <button id="ext-gen3506" class="x-btn-text" type="button">Cancel</button>
            </em>
        </td>
    </tr>
    </tbody>
</table>