body
{
	width:130px;
	height:100px;
	margin:0;
	padding:0;
}
#GadgetContent
{
	/*Works correct*/
	width: 126px;
	height: 96px;
	/*not work, no space for the border:
	width: 100%;
	height: 100%;*/
	padding: 0px;
	margin: 0px;
	top: 0px;
	left:0px;
	text-align: center;
	font-family: Tahoma;
	font-size: 10pt;
	position: absolute;
	border: solid 2px red;
	background-color:Green;
}

<body>
	<div id="GadgetContent">Hello!</div>
</body>