Question

Here’s a screenshot. It’s probably trivial, but I’m stumped.

Answer 1

It’s a suggestion from my side is that first inspect the things with browsers developer tools & there are many tutorials are related to this type of functionality.

I have created a example for you may be that’s help you. Write like this:

ul{
    border-bottom:1px solid red;
}
ul:after{
    content:'';
    clear:both;
    display:block;
}
li{
    float:left;
    height:50px;
    line-height:50px;
    padding:0 10px;
    border:1px solid red;
    margin-bottom:-1px;
}
li.active{
    border-bottom:1px solid #fff;
} 

Check this http://jsfiddle.net/Tj3LH/