                        课程
                    
                        /前端开发
                        
                            /HTML/CSS
                        
                        /如何用CSS进行网页布局

margin-left和margin-right都设置成10px，为什么右侧比左侧宽出7px???

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width,initial-scale=1.0"/>
    <title>responsive web design</title>
    <link rel="stylesheet" href="css/web.css"/>
</head>
<body>
<div id="wrapper">
    <header>
        这是页眉
        <nav>
            <ul>
                <li><a href="#">火狐</a></li>
                <li><a href="#">火狐</a></li>
                <li><a href="#">火狐</a></li>
                <li><a href="#">火狐</a></li>
                <li><a href="#">火狐</a></li>
            </ul>
        </nav>
    </header>
    <article>这是正文</article>
    <aside>这是边栏</aside>
    <footer>这是页脚</footer>
</div>
</body>
</html>

#wrapper{
    margin-left: auto;margin-right: auto;
    width: 96%;
}
header{
    margin-left: 10px;margin-right: 10px;
    width: 97.9166667%;
    background-color: #8e8e8e;
}
nav{
    margin-top: 26px;margin-left: -10px;
    padding-right: 10px;padding-left: 10px;padding-bottom: 25px;
    width: 72.7083333%;/*698除以960*/
}
nav ul li{
    display: inline-block;
}
aside{
    float: left;
    margin-top: 58px;margin-left: 10px;margin-right: 10px;
    border: 2px solid #e8e8e8;
    width: 22.7083333%;/*218除以960*/
    background-color: #4593fd;
}
article{
    float: right;
    margin-top: 58px;margin-right: 17px;
    width: 72.7083333%;
    background-color: #ff2f4f;
}
footer{
    clear: both;
    margin-top:20px;margin-left: 10px;margin-right: 10px;
    width: 97.9166667%;
    background-color: #292929;
}

/*@media screen and (max-width: 960px) {*/
    /*body{background-color: red;}*/
/*}*/
/*放在css文件最后*/
@media screen and (max-width: 768px) {
    #wrapper,header,nav,footer{margin: 0;width: 768px;}
    nav{padding-top: 10px;border-top: 4px double #bfbfbf;text-align: center;}
    nav ul li a{border-radius:10px;font-size: 40px;line-height: 60px;background-color: #dedede;text-decoration: none;}
    article,aside{width:728px;margin-top: 20px;padding-left: 10px;padding-right: 10px;}
}