7.4 CSS float

The most unpredictable property

Behind the word float, an endless sea of possibilities (and misbehaviors).

float is probably the most difficult CSS concept to grasp. Its behavior can be intriguing, unexpected, and magical. Probably because, of all positioning properties there are, it is the one that most influences its surroundings.

In other words, applying a float not only modifies the element it’s applied upon but also alters its ancestors, siblings, descendants, and following elements.

float can only have one of these 3 values:

  • left and right turns an element into a floating one
  • none removes the floating aspect

When to use float

The purpose of floating an element is to push it to one side and make the text wrap around it.

To explain the behaviour, let’s use a common example: floating an image within a paragraph.

<p>
  <img src="http://placehold.it/150x150">
  The bells of the neighbouring church made a jangling tumult, a cart carelessly driven smashed, amid shrieks and curses, against the water trough up the street.  Sickly yellow lights went to and fro in the houses, and some of the passing cabs flaunted unextinguished lamps. And overhead the dawn was growing brighter, clear and steady and calm.
</p>

It was while the curate had sat and talked so wildly to me under the hedge in the flat meadows near Halliford, and while my brother was watching the fugitives stream over Westminster Bridge, that the Martians had resumed the offensive. So far as one can ascertain from the conflicting accounts that have been put forth, the majority of them remained busied with preparations in the Horsell pit until nine that night, hurrying on some operation that disengaged huge volumes of green smoke.

The problem when inserting an image within a text is that an image must fit on a single line of text, and will therefore extend the height of the line it’s on. In our case, our image is 150px high.

What we want is to wrap the text around the image:

img{ float: left;}

It was while the curate had sat and talked so wildly to me under the hedge in the flat meadows near Halliford, and while my brother was watching the fugitives stream over Westminster Bridge, that the Martians had resumed the offensive. So far as one can ascertain from the conflicting accounts that have been put forth, the majority of them remained busied with preparations in the Horsell pit until nine that night, hurrying on some operation that disengaged huge volumes of green smoke.

As you can see, the image is pushed to the left, and the text that follows just wraps around the image:

  • first, the text is pushed to the right, next to the image
  • then, when there’s space available below the image, the text will fill that space

What if the text isn’t long enough?

He heard footsteps running to and fro in the rooms, and up and down stairs behind him

The floating image will overflow because it’s higher than its yellow container. And as you can actually see, it even visually breaks this paragraph you’re reading.

I’ve intentionally left this layout error to showcase why floats are unpredictable: they can even break their parent’s siblings!

Because float: left takes the image out of the flow, the yellow paragraph’s height is only the height of its text. In other words, the height of the image isn’t taken into account.

Float = block

Floating elements will have a display: block applied to them automatically, and will mostly behave like blocks:

  • you can set a specific height and width
  • if no height is set, the element’s height is that of the line-height
  • if a width: 100% is applied, it will look like a block-level element

Clearing the float

The clear property allows to push elements after the float. It can only be applied on block elements.

<p>
  <img src="http://placehold.it/150x150">
  <span>He heard footsteps running to and fro in the rooms, and up and down stairs behind him</span>
</p>
img{ float: left;}
span{ clear: left; display: block;}

He heard footsteps running to and fro in the rooms, and up and down stairs behind him

Instead of having the text pushed next to the image, the clear: left pushes the text below the image.

It’s different from having no float or clear at all, as the image is on its own line and not on the same line as the text.

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