Rounded corners are now trivially easy to achieve via border-radius. But that only allows us to cut into the shape. What if we want to connect a shape to another with a rounded outward corner. Much easier to explain with a graphic:

Clean HTML

Of course, on the web, just about anything visual is possible. Worst-case-scenario you can use images. Our goal here, as ever, is to use no images (quick! accessible! easy to update!) and use completely clean semantic HTML (quick! accessible! easy to update!). So here's the markup:

<ul class="tabs group">
  <li class="active"><a href="#one">One</a></li>
  <li><a href="#two">Two</a></li>
  <li><a href="#three">Three</a></li>
  <li><a href="#three">Four</a></li>
</ul>

A class of active indicates which tab reflects the current page.

How this is going down

The reason this is tricky is that we need a shape to stick out of the tab element. To get this done while keeping our markup clean, we'll use pseudo elements. If you need a refresher, you can learn about them here and here. Essentially, they can add extra elements to the page that we can style, directly through CSS. Every element can have two - :before and :after. Ultimately we'll be using four per tab, which is possible because each tab is make from two elements, the list item and the anchor link.

Let's visualize this step by step, without looking out any code just yet.

1) Natural State

2) Float

3) Same Size

4) Just one

5) Circles

We'll use two of our four available pseudo elements to place circles on the bottom left and bottom right of the tab.

6) Squares

7) Colorize the tab and content

8) Colorize the pseudo elements

9) Stacking

10) No borders

11) Finishing

CSS

This is a big ass block of CSS, but I've tried to comment it up so that each part makes sense.

.tabs li {
  /* Makes a horizontal row */
  float: left; 

  /* So the psueudo elements can be
     abs. positioned inside */
  position: relative;
}
.tabs a {
  /* Make them block level
     and only as wide as they need */
  float: left;
  padding: 10px 40px;
  text-decoration: none;

  /* Default colors */
  color: black;
  background: #ddc385; 

  /* Only round the top corners */
  -webkit-border-top-left-radius: 15px;
  -webkit-border-top-right-radius: 15px;
  -moz-border-radius-topleft: 15px;
  -moz-border-radius-topright: 15px;
  border-top-left-radius: 15px;
  border-top-right-radius: 15px;
}
.tabs .active {
  /* Highest, active tab is on top */
  z-index: 3;
}
.tabs .active a {
  /* Colors when tab is active */
  background: white;
  color: black;
}
.tabs li:before, .tabs li:after,
.tabs li a:before, .tabs li a:after {
  /* All pseudo elements are
     abs. positioned and on bottom */
  position: absolute;
  bottom: 0;
}
/* Only the first, last, and active
   tabs need pseudo elements at all */
.tabs li:last-child:after,   .tabs li:last-child a:after,
.tabs li:first-child:before, .tabs li:first-child a:before,
.tabs .active:after,   .tabs .active:before,
.tabs .active a:after, .tabs .active a:before {
  content: "";
}
.tabs .active:before, .tabs .active:after {
  background: white; 

  /* Squares below circles */
  z-index: 1;
}
/* Squares */
.tabs li:before, .tabs li:after {
  background: #ddc385;
  width: 10px;
  height: 10px;
}
.tabs li:before {
  left: -10px;
}
.tabs li:after {
  right: -10px;
}
/* Circles */
.tabs li a:after, .tabs li a:before {
  width: 20px;
  height: 20px;
  /* Circles are circular */
  -webkit-border-radius: 10px;
  -moz-border-radius:    10px;
  border-radius:         10px;
  background: #222;

  /* Circles over squares */
  z-index: 2;
}
.tabs .active a:after, .tabs .active a:before {
  background: #ddc385;
}
/* First and last tabs have different
   outside color needs */
.tabs li:first-child.active a:before,
.tabs li:last-child.active a:after {
  background: #222;
}
.tabs li a:before {
  left: -20px;
}
.tabs li a:after {
  right: -20px;
}

That's that!

View Demo   Download Files

Should work in just about any decent browser and also IE 9 and up. Should also fall back fine (just no round-outs) in older browsers.

Steve Smith Method

About the exact time as I was creating this to include in a talk I was doing about pseudo elements. Steve Smith of Ordered List published a very similar method. Steve's method has borders around the tabs, mine has tabs butted up against each other. Both cool if you ask me.