HTML超链接在导航栏中不起作用

我的HTML超链接在网页的导航栏上不起作用。我逐渐转移了我的内容stylesheets，scripts发现添加之后CSS file，超链接无法定向到该网页。最初，我发现导航菜单与滑块部分重叠（在先前的StackOverflow问题中提到的问题），但是我为导航菜单本身指定了较高的z-index。抱歉，如果代码很长，但是我必须给所有代码以查看问题出在哪里。请尝试使响应成为实际的解决方法，而不是仅仅删除其中一个元素以停止覆盖。

的HTML

<div id="container">
    <header>
        <h1><b>Seattle</b>&Metropolitan</h1>
        <nav>
            <ul>
                <li><a href="about.html">About</a></li>
                <li>Buildings</li>
                <li id="contact">Contact Us</li>
            </ul>
        </nav>
    </header>
</div>
<div class="image-section">
    <img src="img/seattleskyline.jpg" alt="Seattle Skyline" id="center-image" />
    <div id="caption">A panoramic view of 1201 Third Avenue at night</div>
    <button id="sliderLeft" onclick="left();"></button>
    <button id="sliderRight" onclick="right();"></button>
</div>
<br><br>
<div class="content">
    Seattle's history can be traced back to the 1850s, when pioneers and natives started building a great city filled with a diverse culure, beautiful scenery, and a vibrant enviornment. The metropolitan area of Seattle now is a high-tech hub, in which four Fortune 500 companies reside: <a href="http://www.amazon.com/" alt="Amazon Website"><b>Amazon.com (#49)</b></a>, <a href="http://www.starbucks.com" alt="Starbucks Website"><b>Starbucks (#208)</b></a>, <a href="http://shop.nordstrom.com" alt="Nordstrom Website"><b>Nordstrom (#227)</b></a>, and <a href="http://www.expeditors.com" alt="Expeditors Website"><b>Expeditors International (#428)</b></a>. 
</div>

的CSS

@charset "utf-8";

body
{
    margin: 0;
    padding: 0;
    font-family: Open Sans, sans-serif;
}

#container
{
    width: 75%;
    margin-left: auto;
    margin-right: auto;
}

header h1
{
    font-size: 38px;
    float: left;
    font-weight: 100;
}

header nav ul
{
    list-style-type: none;
    margin: 0;
    vertical-align: middle;
    line-height: normal;
    float: right;
    z-index: 999;
}

header nav ul li
{
    line-height: 105px;
    display: inline;
    padding: 45px;
    font-size: 22px;
    font-weight: 100;
}

header nav ul li a
{
    color: black;
    text-decoration: none;
}

#center-image
{
    width: 100%;
    height: 480px;
}

#contact
{
    padding-right: 0;
}

.image-section
{
    margin-left: auto;
    margin-right: auto;
    width: 75%;
    position: relative;
    text-align: center;
}

.image-section #caption
{
    position: absolute;
    display: none;
    bottom: 4px;
    width: 100%;
    text-align: center;
    color: white;
    background: #474747;
    height: 50px;
    line-height: 50px;
    opacity: 0.8;
    font-size: 20px;
}

.image-section button
{
    outline: 0;
}

.image-section #sliderLeft
{
    position: absolute;
    display: none;
    width: 25px;
    height: 100px;
    top: 50%;
    margin-bottom: 50px;
    left: 0;
    opacity: 0.7;
    border: 0;
}

.image-section #sliderRight
{
    position: absolute;
    display: none;
    width: 25px;
    height: 100px;
    top: 50%;
    margin-bottom: 50px;
    right: 0;
    opacity: 0.7;
    border: 0;
}

.content
{
    margin-left: auto;
    margin-right: auto;
    width: 75%;
    font-size: 18px;
    line-height: 35px;
}

.content a
{
    text-decoration: none;
    color: black;
}

.content a:visited
{
    color: black;
}