Floating elements within a div, floats outside of div. Why?

Question

Say you have a div, give it a definite width and put elements in it, in my case an img and another div.

The idea

Answer 1

Reason

The problem is that floating elements are out-of-flow:

An element is called out of flow if it is floated, absolutely positioned, or is the root element.

Therefore, they don't impact surrounding elements as an in-flow element would.

This is explained in 9.5 Floats:

Since a float is not in the flow, non-positioned block boxes created before and after the float box flow vertically as if the float did not exist. However, the current and subsequent line boxes created next to the float are shortened as necessary to make room for the margin box of the float.

html {
  width: 550px;
  border: 1px solid;
}
body {
  font-family: sans-serif;
  color: rgba(0,0,0,.15);
}
body:after {
  content: '';
  display: block;
  clear: both;
}
div {
  position: relative;
}
div:after {
  font-size: 200%;
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  text-align: center;
}
.block-sibling {
  border: 3px solid green;
}
.block-sibling:after {
  content: 'Block sibling';
  color: green;
}
.float {
  float: left;
  border: 3px solid red;
  height: 90px;
  width: 150px;
  z-index: 1;
}
.float:after {
  content: 'Float';
  color: red;
}

<div class="float"></div>
<div class="block-sibling">
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam viverra nec consectetur ante hendrerit. Donec et mollis dolor. Praesent et diam eget libero egestas mattis sit amet vitae augue. Nam tincidunt congue enim, ut porta lorem lacinia consectetur. Donec ut libero sed arcu vehicula ultricies a non tortor.
</div>

This is also specified in 10.6 Calculating heights and margins. For "normal" blocks,

Only children in the normal flow are taken into account (i.e., floating boxes and absolutely positioned boxes are ignored […])

html {
  width: 550px;
  border: 1px solid;
}
body {
  font-family: sans-serif;
  color: rgba(0,0,0,.15);
}
body:after {
  content: '';
  display: block;
  clear: both;
}
div {
  position: relative;
}
div:after {
  font-size: 200%;
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  text-align: center;
}
.block-parent {
  border: 3px solid blue;
}
.block-parent:after {
  content: 'Block parent';
  color: blue;
}
.float {
  float: left;
  border: 3px solid red;
  height: 130px;
  width: 150px;
}
.float:after {
  content: 'Float';
  color: red;
}

<div class="block-parent">
  <div class="float"></div>
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam viverra nec consectetur ante hendrerit.
</div>

Hacky solution: clearance

A way to solve the problem is forcing some in-flow element to be placed below all floats. Then, the height of the parent will grow to wrap that element (and thus the floats too).

This can be achieved using the clear property:

This property indicates which sides of an element's box(es) may not be adjacent to an earlier floating box.

html {
  width: 550px;
  border: 1px solid;
}
body {
  font-family: sans-serif;
  color: rgba(0,0,0,.15);
}
body:after {
  content: '';
  display: block;
  clear: both;
}
div {
  position: relative;
}
div:after {
  font-size: 200%;
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  text-align: center;
}
.block-parent {
  border: 3px solid blue;
}
.block-parent:after {
  content: 'Block parent';
  color: blue;
}
.float {
  float: left;
  border: 3px solid red;
  height: 84px;
  width: 150px;
}
.float:after {
  content: 'Float';
  color: red;
}
.clear {
  clear: both;
  text-align: center;
  height: 37px;
  border: 3px dashed pink;
}
.clear:after {
  position: static;
  content: 'Block sibling with clearance';
  color: pink;
}

<div class="block-parent">
  <div class="float"></div>
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam viverra.
  <div class="clear"></div>
</div>

So a solution is adding an empty element with clear: both as the last sibling of the floats

<div style="clear: both"></div>

However, that is not semantic. So better generate a pseudo-element at the end of the parent:

.clearfix::after {
  clear: both;
  display: block;
}

There are multiple variants of this approach, e.g. using the deprecated single colon syntax :after to support old browsers, or using other block-level displays like display: table.

Solution: BFC roots

There is an exception to the problematic behavior defined at the beginning: if a block element establishes a Block Formatting Context (is a BFC root), then it will also wrap its floating contents.

According to 10.6.7 'Auto' heights for block formatting context roots,

If the element has any floating descendants whose bottom margin edge is below the element's bottom content edge, then the height is increased to include those edges.

html {
  width: 550px;
  border: 1px solid;
}
body {
  font-family: sans-serif;
  color: rgba(0,0,0,.15);
}
body:after {
  content: '';
  display: block;
  clear: both;
}
div {
  position: relative;
}
div:after {
  font-size: 200%;
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  text-align: center;
}
.block-parent {
  border: 3px solid blue;
}
.block-parent.bfc-root:after {
  content: 'BFC parent';
  color: blue;
}
.float {
  float: left;
  border: 3px solid red;
  height: 127px;
  width: 150px;
}
.float:after {
  content: 'Float';
  color: red;
}
.bfc-root {
  overflow: hidden;
}

<div class="block-parent bfc-root">
  <div class="float"></div>
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam viverra nec consectetur ante hendrerit.
</div>

Additionally, as explained 9.5 Floats, BFC roots are also useful because of the following:

The border box of a table, a block-level replaced element, or an element in the normal flow that establishes a new block formatting context […] must not overlap the margin box of any floats in the same block formatting context as the element itself.

html {
  width: 550px;
  border: 1px solid;
}
body {
  font-family: sans-serif;
  color: rgba(0,0,0,.15);
}
body:after {
  content: '';
  display: block;
  clear: both;
}
div {
  position: relative;
}
div:after {
  font-size: 200%;
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  text-align: center;
}
.block-sibling {
  border: 3px solid green;
}
.block-sibling.bfc-root:after {
  content: 'BFC sibling';
  color: green;
}
.float {
  float: left;
  border: 3px solid red;
  height: 90px;
  width: 150px;
  z-index: 1;
}
.float:after {
  content: 'Float';
  color: red;
}
.bfc-root {
  overflow: hidden;
}

<div class="float"></div>
<div class="block-sibling bfc-root">
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam viverra nec consectetur ante hendrerit. Donec et mollis dolor. Praesent et diam eget libero egestas mattis sit amet vitae augue. Nam tincidunt congue enim, ut porta lorem lacinia consectetur.
</div>

A block formatting context is established by

• Block boxes with overflow other than visible, e.g. hidden

  .bfc-root {
    overflow: hidden;
    /* display: block; */
  }
  

• Block containers that are not block boxes: when display is set to inline-block, table-cell or table-caption.

  .bfc-root {
    display: inline-block;
  }
  

• Floating elements: when float is set to left or right.

  .bfc-root {
    float: left;
  }
  

• Absolutely positioned elements: when position is set to absolute or fixed.

  .bfc-root {
    position: absolute;
  }
  

Note those may have undesired collateral effects, like clipping overflowing content, calculating auto widths with the shrink-to-fit algorithm, or becoming out-of-flow. So the problem is that it's not possible to have an in-flow block-level element with visible overflow that establishes a BFC.

Display L3 addresses these issues:

Created the flow and flow-root inner display types to better express flow layout display types and to create an explicit switch for making an element a BFC root. (This should eliminate the need for hacks like ::after { clear: both; } and overflow: hidden […])

Sadly, there is no browser support yet. Eventually we may be able to use

.bfc-root {
  display: flow-root;
}

Answer 2

W3Schools recommendation:

put overflow: auto on parent element and it will "color" whole background including elements margins. Also floating elements will stay inside of border.

http://www.w3schools.com/css/tryit.asp?filename=trycss_layout_clearfix

Answer 3

As Lucas says, what you are describing is the intended behaviour for the float property. What confuses many people is that float has been pushed well beyond its original intended usage in order to make up for shortcomings in the CSS layout model.

Have a look at Floatutorial if you'd like to get a better understanding of how this property works.

Answer 4

Put your floating div(s) in a div and in CSS give it overflow:hidden;
it will work fine.