<div id="calculadora">
<fieldset>
    <legend>Calculadora</legend>
    <div id="erro"></div>
    <label>
        <span>Operador</span>
        <select id="operador">
            <option></option>
            <option value="somar">Somar</option>
            <option value="diminuir">Diminuir</option>
            <option value="dividir">Dividir</option>
            <option value="multiplicar">Multiplicar</option>
        </select>
    </label>
    <label>
        <span>Valor 1</span>
        <input type="text" id="valor" />
    </label>
    <label>
        <span>Valor 2</span>
        <input type="text" id="valor2" />
    </label>
    <input type="button" onclick="calcular()" id="calc" value="Calcular" />
    <div id="showResult"></div>
</fieldset>