You could use border-top-left-radius and border-top-right-radius properties to round the corners on the box according to the box's height (and added borders).
Then add a border to top/right/left sides of the box to achieve the effect.
Here you go:
.half-circle {
width: 200px;
height: 100px; /* as the half of the width */
background-color: gold;
border-top-left-radius: 110px; /* 100px of height + 10px of border */
border-top-right-radius: 110px; /* 100px of height + 10px of border */
border: 10px solid gray;
border-bottom: 0;
}
Alternatively, you could add box-sizing: border-box to the box in order to calculate the width/height of the box including borders and padding.
.half-circle {
width: 200px;
height: 100px; /* as the half of the width */
border-top-left-radius: 100px;
border-top-right-radius: 100px;
border: 10px solid gray;
border-bottom: 0;
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
}
UPDATED DEMO. (Demo without background color)