Simple JavaScript Percentage Calculator

<h1>Find Percentage of a Number</h1>
<ul>
<li>
<label for="base">Base:</label>
<input type="text" id="base" />
</li>
<li>
<label for="percent">Percent:</label>
<input type="text" id="percent" />
</li>
<li>
<p><span id="total">Total?</span></p>
</li>
<li>
<input type="submit" id="submit" value="Calculate?" onclick="mainInit()" />
</li>
</ul>
<script type="text/javascript">

function mainInit() {
    if(document.getElementById) {
        if(document.getElementById("submit").onclick) {
            calculate();
        }
    }
    else {
        alert("Sorry, your browser does not support JavaScript.");
    }
}

function calculate() {
    var a = parseFloat(document.getElementById("base").value);
    var b = parseFloat(document.getElementById("percent").value);
    var total = document.getElementById("total");
    total.innerHTML = (a * (b/100)).toFixed(2);
}
</script>