.brtop只是增加了上限.

似乎没有做任何事情,我做错了什么？

/* html5doctor.com Reset v1.6.1 - http://cssreset.com */
html,body,div,span,object,iframe,h1,h2,h3,h4,h5,h6,p,blockquote,pre,abbr,address,cite,code,del,dfn,em,img,ins,kbd,q,samp,small,strong,sub,sup,var,b,i,dl,dt,dd,ol,ul,li,fieldset,form,label,legend,table,caption,tbody,tfoot,thead,tr,th,td,article,aside,canvas,details,figcaption,figure,footer,header,hgroup,menu,nav,section,summary,time,mark,audio,video{margin:0;padding:0;border:0;outline:0;font-size:100%;vertical-align:baseline;background:transparent}body{line-height:1}article,section{display:block}nav ul{list-style:none}blockquote,q{quotes:none}blockquote:before,blockquote:after,q:before,q:after{content:none}a{margin:0;padding:0;font-size:100%;vertical-align:baseline;background:transparent}ins{background-color:#ff9;color:#000;text-decoration:none}mark{background-color:#ff9;color:#000;font-style:italic;font-weight:bold}del{text-decoration:line-through}abbr[title],dfn[title]{border-bottom:1px dotted;cursor:help}table{border-collapse:collapse;border-spacing:0}hr{display:block;height:1px;border:0;border-top:1px solid #ccc;margin:1em 0;padding:0}input,select{vertical-align:middle}

/*  ^^CSS_RESET^^ --- vv CSSforsite vv */

form{
	width:500px;
	padding:1.5em;
	overflow:hidden;
	border:1px solid red;
}
fieldset{
	float:left;
	border:none;
	margin-left:1em;
}

.brtop{
margin-top:5px;
}

input{
}

<form name="messageform">
		<fieldset>
			<label for="username" >Name:</label><br/>
			<input type="text" name="username"/><br/>
			<label for="email"class="brtop">Email:</label><br/>
			<input type="text" name="email"/>
		</fieldset>
		<fieldset>
			<label>Message:</label><br/>
			<textarea name="message" cols="20" rows="4"></textarea>
		</fieldset>		
	</form>