Last Updated: September 09, 2019
·
12.56K
· nilgradisnik

CSS inverted border radius

Everybody knows how to make rounded borders, it's right there in the CSS3 spec. Ever wondered how to do borders with radius inverted in pure CSS? I was facing this challenge a while ago and I just wanted to share the results.

You start off witht this HTML structure

<div class="outer">
  <div class="inner">
    <i class="top left"></i>
    <i class="top right"></i>
    <div class="content"></div>
    <i class="bottom right"></i>
    <i class="bottom left"></i>
  </div>
</div>

<small>As you can see it takes quite a few extra DOM elements to achieve this.</small>

The following CSS will do the trick

.outer {
  overflow: hidden;
}
.inner {
  border: 1px solid #888;
}
.inner i {
  width: 40px;
  height: 40px;
  border: 1px solid #888;
  border-radius: 50%;
  background-color: #fff;
}
.inner .top {
  margin-top: -20px;
}
.inner .bottom {
  margin-top: -20px;
  margin-bottom: -22px;
}
.inner .left {
  float: left;
  margin-left: -20px;
}
.inner .right {
  float: right;
  margin-right: -20px;
}
.content {
  min-height: 80px;
}

You can control the border radius by changing the width and the height of the .inner i and adjust the margins accordingly, which is half of the width/height. Except the margin-bottom fo the .bottom element needs to be substracted by twice the number of pixles of the .inner border thickness which is this case is 2px. 1px on top and 1px on the bottom.

One thing to note about this is that you will need a solid background color of the parent container. As you can see the .inner i has a backgournd-color set, so this won't work if you are placing this on a background image or a gradient.

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6 Responses
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Like for the clever tip and the clever recruiting tactic. Genius on two levels.

over 1 year ago ·

this is amazing stuff - many thanks for the tip!

over 1 year ago ·

You could use :before and :after with content: ""; instead of the four extra i elements (then you would only need outer and inner (and you might even be able to get rid of one of them if content could stand in for either of them).

over 1 year ago ·

Ha. We all spent forever using this technique for rounded corners, until border-radius finally arrived and we threw off our <div class="top left corner"> shackles.

Freedom! ...now then, I feel like this element could really use some some inverted rounde.... ah crap.

over 1 year ago ·

@svieira That was actually my first thought. But you can only apply only one :before and :after per element so I'm not sure that would work. And you need the .outer because of the overflow: hidden. I'd love to see your solution in action though.

over 1 year ago ·

Absolutely clever trick. Thanks for posting that. I may have to try that out on my next site.

over 1 year ago ·