index.html

<nav>
  <h1>Logo</h1>
<a>Lorem</a><a class='active'>Lorem Ipsum</a>
<a>Ipsum Da Tada!</a>
<div class='filler'></div>
</nav>

MYppqm.markdown

MYppqm

A Pen by Captain Anonymous on CodePen.

License.

style.css

body {
    margin:0;
    padding:0;
    background: url('http://lorempixel.com/output/people-q-c-640-480-1.jpg');
    background-size: cover;
}

nav {
  position: relative;
  display: flex;
  flex-flow: row nowrap;
  justify-content: flex-start;
}

nav > h1 {
  margin: 0;
}

nav > * {
  background: rgba(255,255,255,0.7);
  padding: 10px 15px 30px;
  min-width: 100px;
  text-align: center;
  position:relative;
  overflow: hidden;
  z-index:1;
}

a.active {
  background: none;
}

a.active:after {
  content:'';
  position:absolute;
  left: calc(50% - 10px);
  width: 20px;
  padding-bottom: 20px;
  bottom: -10px;
  background: none;
  border-radius: 50%;
  box-shadow: 0px 0px 0px 150px rgba(255,255,255,0.7);
  z-index: -1;
}

div.filler {
  flex: 1;
  align-self: flex-stretch; 
}