Bootstrap includes dozens of utilities—classes with a single purpose. They're designed to reduce the frequency of highly repetitive declarations in your CSS while allowing for quick and easy development.
## Contents
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## Spacing
Assign `margin` or `padding` to an element or a subset of its sides with shorthand classes. Includes support for individual properties, all properties, and vertical and horizontal properties. All classes are multiples on the global default value, `1rem`.
The classes are named using the format: `{property}-{sides}-{size}`
Where *property* is one of:
*`m` - for classes that set `margin`
*`p` - for classes that set `padding`
Where *sides* is one of:
*`t` - for classes that set `margin-top` or `padding-top`
*`b` - for classes that set `margin-bottom` or `padding-bottom`
*`l` - for classes that set `margin-left` or `padding-left`
*`r` - for classes that set `margin-right` or `padding-right`
*`x` - for classes that set both `*-left` and `*-right`
*`y` - for classes that set both `*-top` and `*-bottom`
*`a` - for classes that set a `margin` or `padding` on all 4 sides of the element
Where *size* is one of:
*`0` - for classes that eliminate the `margin` or `padding` by setting it to `0`
*`1` - (by default) for classes that set the `margin` or `padding` to `$spacer-x` or `$spacer-y`
*`2` - (by default) for classes that set the `margin` or `padding` to `$spacer-x * 1.5` or `$spacer-y * 1.5`
*`3` - (by default) for classes that set the `margin` or `padding` to `$spacer-x * 3` or `$spacer-y * 3`
Additionally, Bootstrap also includes an `.m-x-auto` class for horizontally centering fixed-width block level content by setting the horizontal margins to `auto`.
Convey meaning through color with a handful of emphasis utility classes. These may also be applied to links and will darken on hover just like our default link styles.
{% example html %}
<pclass="text-muted">Fusce dapibus, tellus ac cursus commodo, tortor mauris nibh.</p>
<pclass="text-primary">Nullam id dolor id nibh ultricies vehicula ut id elit.</p>
<pclass="text-success">Duis mollis, est non commodo luctus, nisi erat porttitor ligula.</p>
<pclass="text-info">Maecenas sed diam eget risus varius blandit sit amet non magna.</p>
<pclass="text-warning">Etiam porta sem malesuada magna mollis euismod.</p>
<pclass="text-danger">Donec ullamcorper nulla non metus auctor fringilla.</p>
{% endexample %}
Contextual text classes also work well on anchors with the provided hover and focus states.
{% example html %}
<ahref="#"class="text-muted">Muted link</a>
<ahref="#"class="text-primary">Primary link</a>
<ahref="#"class="text-success">Success link</a>
<ahref="#"class="text-info">Info link</a>
<ahref="#"class="text-warning">Warning link</a>
<ahref="#"class="text-danger">Danger link</a>
{% endexample %}
Similar to the contextual text color classes, easily set the background of an element to any contextual class. Anchor components will darken on hover, just like the text classes.
{% example html %}
<divclass="bg-primary">Nullam id dolor id nibh ultricies vehicula ut id elit.</div>
<divclass="bg-success">Duis mollis, est non commodo luctus, nisi erat porttitor ligula.</div>
<divclass="bg-info">Maecenas sed diam eget risus varius blandit sit amet non magna.</div>
<divclass="bg-warning">Etiam porta sem malesuada magna mollis euismod.</div>
<divclass="bg-danger">Donec ullamcorper nulla non metus auctor fringilla.</div>
<divclass="bg-inverse">Cras mattis consectetur purus sit amet fermentum.</div>
{% endexample %}
{% callout info %}
#### Dealing with specificity
Sometimes contextual classes cannot be applied due to the specificity of another selector. In some cases, a sufficient workaround is to wrap your element's content in a `<div>` with the class.
Use a generic close icon for dismissing content like modals and alerts. **Be sure to include text for screen readers**, as we've done with `aria-label`.
These utility classes float an element to the left or right, or disable floating, based on the current viewport size using the [CSS `float` property](https://developer.mozilla.org/en-US/docs/Web/CSS/float). `!important` is included to avoid specificity issues. These use the same viewport width breakpoints as the grid system.
Two similar non-responsive mixins (`pull-left` and `pull-right`) are also available.
{% example html %}
<divclass="pull-xs-left">Float left on all viewport sizes</div><br>
<divclass="pull-xs-right">Float right on all viewport sizes</div><br>
<divclass="pull-xs-none">Don't float on all viewport sizes</div><br>
<divclass="pull-sm-left">Float left on viewports sized SM (small) or wider</div><br>
<divclass="pull-md-left">Float left on viewports sized MD (medium) or wider</div><br>
<divclass="pull-lg-left">Float left on viewports sized LG (large) or wider</div><br>
<divclass="pull-xl-left">Float left on viewports sized XL (extra-large) or wider</div><br>
Easily clear `float`s by adding `.clearfix`**to the parent element**. Utilizes [the micro clearfix](http://nicolasgallagher.com/micro-clearfix-hack/) as popularized by Nicolas Gallagher. Can also be used as a mixin.
The `.pos-f-t` class can be used to easily position elements at the top of the viewport and make them as wide as the viewport. **Be sure you understand the ramifications of fixed-position elements within your project.** Here's how the class is defined:
The `.invisible` class can be used to toggle only the visibility of an element, meaning its `display` is not modified and the element can still affect the flow of the document.
{% highlight html %}
<divclass="invisible">...</div>
{% endhighlight %}
{% highlight scss %}
// Class
.invisible {
visibility: hidden;
}
// Usage as a mixin
.element {
@include invisible;
}
{% endhighlight %}
## Screen readers and keyboard users
Hide an element to all devices **except screen readers** with `.sr-only`. Combine `.sr-only` with `.sr-only-focusable` to show the element again when it's focused (e.g. by a keyboard-only user). Can also be used as mixins.
{% comment %}
Necessary for following [accessibility best practices](../getting-started/#accessibility).
{% endcomment %}
{% highlight html %}
<aclass="sr-only sr-only-focusable"href="#content">Skip to main content</a>
{% endhighlight %}
{% highlight scss %}
// Usage as a mixin
.skip-navigation {
@include sr-only;
@include sr-only-focusable;
}
{% endhighlight %}
## Image replacement
Utilize the `.text-hide` class or mixin to help replace an element's text content with a background image.
{% highlight html %}
<h1class="text-hide">Custom heading</h1>
{% endhighlight %}
{% highlight scss %}
// Usage as a mixin
.heading {
@include text-hide;
}
{% endhighlight %}
## Responsive embeds
Allow browsers to determine video or slideshow dimensions based on the width of their containing block by creating an intrinsic ratio that will properly scale on any device.
Rules are directly applied to `<iframe>`, `<embed>`, `<video>`, and `<object>` elements; optionally use an explicit descendant class `.embed-responsive-item` when you want to match the styling for other attributes.
**Pro-Tip!** You don't need to include `frameborder="0"` in your `<iframe>`s as we override that for you.
