If there are contents within the div that has the curved corners, you have to set overflow: hidden because otherwise the child div's overflow can give the impression that the border-radius isn't working.

<!-- This will look like the border-radius isn't working-->
<div style="border: 1px solid black; border-radius: 10px;">
  <div style="background: red;">
      text!
  </div>
</div>

<!-- but here the contents properly fit within the rounded div -->
<div style="border: 1px solid black; border-radius: 10px; overflow: hidden;">
  <div style="background: red;">
      text!
  </div>
</div>