<table id="calculator">
    <tr>
        <th id="output" colspan="4"></th>
    </tr>
    <tr>
        <th class="tile">7</th>
        <th class="tile">8</th>
        <th class="tile">9</th>
        <th class="tile action" id="plus">+</th>
    </tr>
    <tr>
        <th class="tile">6</th>
        <th class="tile">5</th>
        <th class="tile">4</th>
        <th class="tile action" id="minus">-</th>
    </tr>
    <tr>
        <th class="tile">3</th>
        <th class="tile">2</th>
        <th class="tile">1</th>
        <th class="tile action" id="mul">*</th>
    </tr>
    <tr>
        <th class="tile action" id="cancel">C</th>
        <th class="tile">0</th>
        <th class="tile action" id="equals">=</th>
        <th class="tile action" id="divide">/</th>
    </tr>
</table>