<!DOCTYPE html>
<style>
    div {
        width: 300px;
        height: 50px;
        background: green;
        border: 10px solid;
        border-radius: 20px;
        box-sizing: border-box;
    }
</style>

<p>You should see four green boxes with rounded corners.</p>

blue border
<div style="border-color:blue"></div>
dark green border
<div style="border-color:rgba(0,0,0,0.5)"></div>
no border
<div style="border-color:transparent"></div>
no border
<div style="border-color:transparent"></div>
