<style>
div {
    width:100px;
    height:100px;
    border: 2px solid black;
}
.control {
    border-radius:50px;    
}
.pixels {
    border-radius:calc(50px);
}
.percent {
    border-radius:calc(50%);
}
.both {
    /* Note that 100% is 104px, so 25% + 25px = 51px*/
    border-radius:calc(25% + 25px);
}
</style>

This test should have four circles.

<div class="control"></div>
<div class="pixels"></div>
<div class="percent"></div>
<div class="both"></div>