Agora SIIIIIIM!
<!DOCTYPE html>
<html>
<head>
<style>
div.container
{
box-sizing:border-box;
-moz-box-sizing:border-box; /* Firefox */
-webkit-box-sizing:border-box; /* Safari */
width:100%;
padding: 5px;
border:1px solid blue;
}
div input
{
box-sizing:border-box;
-moz-box-sizing:border-box; /* Firefox */
-webkit-box-sizing:border-box; /* Safari */
padding: 5px;
width:100%;
border:1px solid red;
}
</style>
</head>
<body>
<div class="container">
<input type="text">
</div>
</body>
</html>