Calculadora

<!DOCTYPE html>
<html>
<head>
	<title></title>
	<style type="text/css">
		table, td,tr{
			border:1px solid black;
			margin: auto;
		}
		td{
			width: 50px;
			height: 50px;
			text-align: center;
		}
		td:active{
			background-color: red;
		}
	</style>
</head>
<body>

	<table>
		<tr>
			<td id="a0" colspan="4" style="background-color: black; color: white;">

			</td>
		</tr>
		<tr>
			<td id="a1" onclick="marca('1');">1</td>
			<td id="a2"  onclick="marca('2');">2</td>
			<td id="a3" onclick="marca('3');">3</td>
			<td id="mais" onclick="op('+');">+  </td>
		</tr>
		<tr>
			<td id="a4" onclick="marca('4');">4</td>
			<td id="a5" onclick="marca('5');">5</td>
			<td id="a6" onclick="marca('6');">6</td>
			<td id="menos" onclick="op('-');">-</td>
		</tr>
		<tr>
			<td id="a7"  onclick="marca('7');">7</td>
			<td id="a8"  onclick="marca('8');">8</td>
			<td id="a9" onclick="marca('9');">9</td>
			<td id="div" onclick="op('/')">/</td>
		</tr>
		<tr>
			<td id="pont" onclick="marca('.');">.</td>
			<td id="a00" onclick="marca('0');">0</td>
			<td id="limpa" onclick="limpa()">C</td>
			<td id="vz" onclick="op('*');">*</td>
		</tr>
		<tr>
			<td id="a10" onclick="op('=');" colspan="4" style="text-align: center;">=</td>
		</tr>

	</table>

	<script type="text/javascript">
		var resp=0;
		var opera='';
		var LIMPA=0;
		function marca(x){
			if(LIMPA==1) {
				document.getElementById('a0').innerHTML="";
				LIMPA=0;
			}
			document.getElementById('a0').innerHTML=document.getElementById('a0').innerHTML + x;

		}

		function op(y){
			LIMPA=1;
			var v=parseInt(document.getElementById('a0').innerHTML);
			if(opera==''){
				opera=y;
				resp=v;
			}
			else{
				switch (opera) {
					case '+':
						resp=resp+v;
						break;

					case '-':
						resp=resp-v;
						break;
					case '*':
						resp=resp*v;
						break;
				}
			}
			if (y=='=') {
				opera='';
			}
			else
				opera=y;
			document.getElementById('a0').innerHTML=resp;
		}

		function limpa(){
			document.getElementById('a0').innerHTML= "";
			resp=0;
		}
	</script>

</body>
</html>