What are the CSS secrets to a flexible/fluid HTML form?

.hide { display: none ; }
.show { display: inline ; }
fieldset { border: #206ba4 1px solid; }
fieldset legend { margin-top: -.4em; font-size: 20px; font-weight: bold; color:     #206ba4; }
fieldset fieldset   { position: relative; margin-top: 25px; padding-top: 0.75em; background-color: #ebf4fa; }
body { margin: 0; padding: 0; font-family : Verdana; font-size : 12px; text-align: center ;  }
#wrapper { margin: 40px auto 0 auto ; }
#myFieldset {   width: 213px ;}
#area { margin: 20px ;}
#area select {width: 75px ; float:left ;}
#area label {display: block ;   font-size: 1.1em;	font-weight: bold; color: #000000;}
#area #selection {display: block ; }
#makes {margin: 5px ;   }
#models {margin: 5px ;  }

var cars = [
{
"makes"  : "Honda",
"models"  : ['Accord','CRV','Pilot']
},
{
"makes"   :"Toyota",
"models"  : ['Prius','Camry','Corolla']
} ] ;

$(function() {
vehicles = [] ;
for(var i=0; i<cars.length; i++) {
vehicles[cars[i].makes] = cars[i].models ;
}
var options = '' ;
for (var i = 0; i < cars.length; i++) {
options += '<option value="' + cars[i].makes + '">' + cars[i].makes + '</option>';
}
$("#make").html(options);   // populate select box with array

$("#make").bind("click",
function() {
	$("#model").children().remove() ;		// clear select box
	var options = '' ;
	for (var i = 0; i < vehicles[this.value].length; i++) {
 options += '<option value="' + vehicles[this.value][i] + '">' + vehicles[this.value][i] + '</option>';
	}
	$("#model").html(options);   // populate select box with array
	$("#models").addClass("show") ;
} 		// bind function end
);  	// bind end
});

<div id="wrapper">
<fieldset id="myFieldset"><legend>Cars</legend>
<fieldset id="area">
<label>Select Make:</label>
<div id="selection">
<div id="makes">
<select id="make"size="2"></select>
</div>
<div class="hide" id="models">
<select id="model" size="3"></select>
</div>
</div>
</fieldset>
</fieldset>
</div>