What is a clearfix?

The other answers are correct. But I want to add that it is a relic of the time when people were first learning CSS, and abused float to do all their layout. float is meant to do stuff like float images next to long runs of text, but lots of people used it as their primary layout mechanism. Since it wasn't really meant for that, you need hacks like "clearfix" to make it work.

These days display: inline-block is a solid alternative (except for IE6 and IE7), although more modern browsers are coming with even more useful layout mechanisms under names like flexbox, grid layout, etc.

If you don't need to support IE9 or lower, you can use flexbox freely, and don't need to use floated layouts.

It's worth noting that today, the use of floated elements for layout is getting more and more discouraged with the use of better alternatives.

• display: inline-block - Better
• Flexbox - Best (but limited browser support)

Flexbox is supported from Firefox 18, Chrome 21, Opera 12.10, and Internet Explorer 10, Safari 6.1 (including Mobile Safari) and Android's default browser 4.4.

For a detailed browser list see: https://caniuse.com/flexbox.

(Perhaps once its position is established completely, it may be the absolutely recommended way of laying out elements.)

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A clearfix is a way for an element to automatically clear its child elements, so that you don't need to add additional markup. It's generally used in float layouts where elements are floated to be stacked horizontally.

The clearfix is a way to combat the zero-height container problem for floated elements

A clearfix is performed as follows:

.clearfix::after {
   content: " "; /* Older browser do not support empty content */
   visibility: hidden;
   display: block;
   height: 0;
   clear: both;
}

Or, if you don't require IE<8 support, the following is fine too:

.clearfix::after {
  content: "";
  display: table;
  clear: both;
}

Normally you would need to do something as follows:

<div>
    <div style="float: left;">Sidebar</div>
    <div style="clear: both;"></div> <!-- Clear the float -->
</div>

With clearfix, you only need the following:

<div class="clearfix">
    <div style="float: left;" class="clearfix">Sidebar</div>
    <!-- No Clearing div! -->
</div>

Read about it in this article - by Chris Coyer @ CSS-Tricks