SOURCE

<div class="warning">
    <div class="snip"></div>
</div>

.warning{
    width: 200px;
    height: 200px;
    border-radius: 50%;
    background-color: red;
    overflow: hidden;
}
.snip{
    position: absolute;
    width: 0;
    height: 0;
    top: 0;
    left: 43px;
    width: 0;
    height: 0;
    border-right: 57px solid transparent;/*100/√3*/
    border-left: 57px solid transparent;
    border-top: 100px solid yellow;
    border-bottom: 100px solid transparent;
    border-radius: 100%;
}