Lo puedes hacer con Jquery (debes bajar la Libreria "Jquery" para que funcione xD)
[TD="class: webkit-line-content"]<!DOCTYPE html>[/TD]
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[TD="class: webkit-line-content"]<html>[/TD]
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[TD="class: webkit-line-content"]<head>[/TD]
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[TD="class: webkit-line-content"] <meta http-equiv="content-type" content="text/html; charset=UTF-8" />[/TD]
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[TD="class: webkit-line-content"] <title>Ejemplo de sumar dos campos</title>[/TD]
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[TD="class: webkit-line-content"] <script type='text/javascript' src='
jquery.js
'></script>[/TD]
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[TD="class: webkit-line-content"] <style type='text/css'>[/TD]
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[TD="class: webkit-line-content"] input {text-align:right;border-color:#808080;}[/TD]
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[TD="class: webkit-line-content"] .text {clear:both;float:left;width:80px;}[/TD]
[TD="class: webkit-line-number"][/TD]
[TD="class: webkit-line-content"] .input {float:left;}[/TD]
[TD="class: webkit-line-number"][/TD]
[TD="class: webkit-line-content"] #total {color:#000;}[/TD]
[TD="class: webkit-line-number"][/TD]
[TD="class: webkit-line-content"] </style>[/TD]
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[TD="class: webkit-line-content"]</head>[/TD]
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[TD="class: webkit-line-content"]<body>[/TD]
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[TD="class: webkit-line-content"] <h1>Sumar dos números con jquery</h1>[/TD]
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[TD="class: webkit-line-content"] <div id='suma'>[/TD]
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[TD="class: webkit-line-content"] <div class='text'>Número 1</div><div class='input'><input type='text' name='valor1' id='valor1'></div>[/TD]
[TD="class: webkit-line-number"][/TD]
[TD="class: webkit-line-content"] <div class='text'>Número 2</div><div class='input'><input type='text' name='valor2' id='valor2'></div>[/TD]
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[TD="class: webkit-line-content"] <div class='text'>Total</div><div class='input'><input type='text' name='total' id='total' disabled='disabled'></div>[/TD]
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[TD="class: webkit-line-content"] </div>[/TD]
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[TD="class: webkit-line-number"][/TD]
[TD="class: webkit-line-content"] <script type='text/javascript'>[/TD]
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[TD="class: webkit-line-content"] // Cada vez que se pulse una tecla, controlamos que sea numerica[/TD]
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[TD="class: webkit-line-content"] $("#suma input").keypress(function(event) {[/TD]
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[TD="class: webkit-line-content"] //obtenemos la tecla pulsada[/TD]
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[TD="class: webkit-line-content"] var valueKey=String.fromCharCode(event.which);[/TD]
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[TD="class: webkit-line-content"] //obtenemos el valor ascii de la tecla pulsada[/TD]
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[TD="class: webkit-line-content"] var keycode=event.which;[/TD]
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[TD="class: webkit-line-number"][/TD]
[TD="class: webkit-line-content"] // Si NO pulsamos un numero, un punto, la tecla suprimir[/TD]
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[TD="class: webkit-line-content"] // la tecla backspace o el simobolo "-" (45), cancelamos la pulsacion[/TD]
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[TD="class: webkit-line-content"] if(valueKey.search('[0-9|\.]')!=0 && keycode!=8 && keycode!=46 && keycode!=45)[/TD]
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[TD="class: webkit-line-content"] {[/TD]
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[TD="class: webkit-line-content"] // anulamos la pulsacion de la tecla[/TD]
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[TD="class: webkit-line-content"] return false;[/TD]
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[TD="class: webkit-line-content"] }[/TD]
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[TD="class: webkit-line-content"] });[/TD]
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[TD="class: webkit-line-content"] // evento que se ejecuta cada vez que se suelte la tecla en cualquiera de[/TD]
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[TD="class: webkit-line-content"] // los tres inputs[/TD]
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[TD="class: webkit-line-content"] $("#suma input").keyup(function(event) {[/TD]
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[TD="class: webkit-line-content"] calcular();[/TD]
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[TD="class: webkit-line-content"] });[/TD]
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[TD="class: webkit-line-content"] // Calculamos la suma de los dos valores[/TD]
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[TD="class: webkit-line-content"] function calcular()[/TD]
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[TD="class: webkit-line-content"] {[/TD]
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[TD="class: webkit-line-content"] var valor1=validarNumero('#valor1');[/TD]
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[TD="class: webkit-line-content"] var valor2=validarNumero('#valor2');[/TD]
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[TD="class: webkit-line-content"] $("#total").val(valor1+valor2);[/TD]
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[TD="class: webkit-line-content"] }[/TD]
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[TD="class: webkit-line-content"] // Funcion para validar que el numero sea correcto, y para cambiar el color[/TD]
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[TD="class: webkit-line-content"] // del marco en caso de error[/TD]
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[TD="class: webkit-line-content"] function validarNumero(id)[/TD]
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[TD="class: webkit-line-content"] {[/TD]
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[TD="class: webkit-line-content"] if($.isNumeric($(id).val()))[/TD]
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[TD="class: webkit-line-content"] {[/TD]
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[TD="class: webkit-line-content"] $(id).css('border-color','#808080');[/TD]
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[TD="class: webkit-line-content"] return parseFloat($(id).val());[/TD]
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[TD="class: webkit-line-content"] }else if($(id).val()==""){[/TD]
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[TD="class: webkit-line-content"] $(id).css('border-color','#808080');[/TD]
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[TD="class: webkit-line-content"] return 0;[/TD]
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[TD="class: webkit-line-content"] }else{[/TD]
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[TD="class: webkit-line-content"] $(id).css('border-color','#f00');[/TD]
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[TD="class: webkit-line-content"] return 0;[/TD]
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[TD="class: webkit-line-content"] }[/TD]
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[TD="class: webkit-line-content"] }[/TD]
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[TD="class: webkit-line-content"] </script>[/TD]
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[TD="class: webkit-line-content"]</body>[/TD]
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[TD="class: webkit-line-content"]</html>
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