Issue with background color of a button after visited state

<https://jsfiddle.net/vivekraj_kr/wxokhpy4/>

how to get a background color on button visited

```
<button type="button" class="size_btn">S</button>
<button type="button" class="size_btn">M</button>
<button type="button" class="size_btn">L</button>
<button type="button" class="size_btn">XL</button>

.size_btn {
height:27px;
width: 27px;
background: none;
border: solid 1px #ccc;
}

.size_btn:visited {
background-color: #479c3d;
}

```