Source

Reboot

Reboot, a collection of element-specific CSS changes in a single file, kickstart Bootstrap to provide an elegant, consistent, and simple baseline to build upon.

Approach

Reboot builds upon Normalize, providing many HTML elements with somewhat opinionated styles using only element selectors. Additional styling is done only with classes. For example, we reboot some <table>styles for a simpler baseline and later provide .table, .table-bordered, and more.

Here are our guidelines and reasons for choosing what to override in Reboot:

Update some browser default values to use rems instead of ems for scalable component spacing.
Avoid margin-top. Vertical margins can collapse, yielding unexpected results. More importantly though, a single direction of marginis a simpler mental model.
For easier scaling across device sizes, block elements should use rems for margins.
Keep declarations of font-related properties to a minimum, using inheritwhenever possible.

Page defaults

The <html>and <body>elements are updated to provide better page-wide defaults. More specifically:

The box-sizingis globally set on every element—including *::beforeand *::after, to border-box. This ensures that the declared width of element is never exceeded due to padding or border.
- No base font-sizeis declared on the <html>, but 16pxis assumed (the browser default). font-size: 1remis applied on the <body>for easy responsive type-scaling via media queries while respecting user preferences and ensuring a more accessible approach.
The <body>also sets a global font-family, line-height, and text-align. This is inherited later by some form elements to prevent font inconsistencies.
For safety, the <body>has a declared background-color, defaulting to #fff.

native font stack

The default web fonts (Helvetica Neue, Helvetica, and Arial) have been dropped in Bootstrap 4 and replaced with a “native font stack” for optimum text rendering on every device and OS. Read more about native font stacks in this Smashing Magazine article .

$font-family-sans-serif:
  // Safari for OS X and iOS (San Francisco)
  -apple-system,
  // Chrome < 56 for OS X (San Francisco)
  BlinkMacSystemFont,
  // Windows
  "Segoe UI",
  // Android
  "Roboto",
  // Basic web fallback
  "Helvetica Neue", Arial, sans-serif,
  // Emoji fonts
  "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol" !default;

This font-familyis applied to the <body>and automatically inherited globally throughout Bootstrap. To switch the global font-family, update $font-family-baseand recompile Bootstrap.

Headings and paragraphs

All heading elements—eg, <h1>—and <p>are reset to have their margin-topremoved. Headings have margin-bottom: .5remadded and paragraphs margin-bottom: 1remfor easy spacing.

heading	example
`<h1></h1>`	h1. Bootstrap heading
`<h2></h2>`	h2. Bootstrap heading
`<h3></h3>`	h3. Bootstrap heading
`<h4></h4>`	h4. Bootstrap heading
`<h5></h5>`	h5. Bootstrap heading
`<h6></h6>`	h6. Bootstrap heading

Lists

All lists— <ul>, <ol>, and <dl>—have their margin-topremoved and a margin-bottom: 1rem. Nested lists have no margin-bottom.

Lorem ipsum dolor sit amet
Consectetur adipiscing elite
Integer molestie lorem at massa
Facilisis in pretium nisl aliquet
Nulla volutpat aliquam velit
- Phasellus iaculis neque
- Purus sodales ultricies
- Vestibulum laoreet porttitor sem
- Ac tristique libero volutpat at
Faucibus porta lacus fringilla vel
Aenean sit amet erat nunc
Eget porttitor lorem

Lorem ipsum dolor sit amet
Consectetur adipiscing elite
Integer molestie lorem at massa
Facilisis in pretium nisl aliquet
Nulla volutpat aliquam velit
Faucibus porta lacus fringilla vel
Aenean sit amet erat nunc
Eget porttitor lorem

For simpler styling, clear hierarchy, and better spacing, description lists have updated margins. <dd>s reset margin-leftto 0and add margin-bottom: .5rem. <dt>s are bolded .

Description lists: A description list is perfect for defining terms.
Euismod: Vestibulum id ligula porta felis euismod semper eget lacinia odio sem.; Donec id elit non mi porta gravida at eget metus.
Malesuada porta: Etiam porta sem malesuada magna mollis euismod.

Preformatted text

The <pre>element is reset to remove its margin-topand use remunits for its margin-bottom.

.example-element {
  margin-bottom: 1rem;
}

tables

Tables are slightly adjusted to style <caption>s, collapse borders, and ensure consistent text-alignthroughout. Additional changes for borders, padding, and more come with the .tableclass .

This is an example table, and this is its caption to describe the contents.
table heading	table heading	table heading	table heading
table cell	table cell	table cell	table cell
table cell	table cell	table cell	table cell
table cell	table cell	table cell	table cell

Forms

Various form elements have been rebooted for simpler base styles. Here are some of the most notable changes:

<fieldset>s have no borders, padding, or margin so they can be easily used as wrappers for individual inputs or groups of inputs.
<legend>s, like fieldsets, have also been restyled to be displayed as a heading of sorts.
<label>s are set to display: inline-blockallow marginto be applied.
<input>s, <select>s, <textarea>s, and <button>s are mostly addressed by Normalize, but Reboot removes their marginand sets line-height: inherit, too.
<textarea>s are modified to only be resizable vertically as horizontal resizing often “breaks” page layout.

These changes, and more, are demonstrated below.

misc elements

address

The <address>element is updated to reset the browser default font-stylefrom italicto normal. line-heightis also now inherited, and margin-bottom: 1remhas been added. <address>s are for presenting contact information for the nearest ancestor (or an entire body of work). Preserve formatting by ending lines with <br>.

Twitter Inc.
1355 Market St, Suite 900
San Francisco, CA 94103
P: (123) 456-7890

Full Name
[email protected]

Blockquote

The default marginon blockquotes is 1em 40px, so we reset that to 0 0 1remfor something more consistent with other elements.

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer posuere erat a ante.

Someone famous in Source Title

Inline elements

The <abbr>element receives basic styling to make it stand out among paragraph text.

Nulla attr vitae elit libero, a pharetra augue.

Summary

The default cursoron summary is text, so we reset that pointerto convey that the element can be interacted with by clicking on it.

Some details

More info about the details.

Even more details

Here are even more details about the details.

HTML5 `[hidden]`attributes

HTML5 adds a new global attribute named[hidden] , which is styled as display: noneby default. Borrowing an idea from PureCSS , we improve upon this default by making [hidden] { display: none !important; }to help prevent its displayfrom getting accidentally overridden. While [hidden]isn't natively supported by IE10, the explicit declaration in our CSS gets around that problem.

<input type="text" hidden>

jQuery incompatibility

[hidden]is not compatible with jQuery's $(...).hide()and $(...).show()methods. Therefore, we don't currently especially endorse [hidden]over other techniques for managing the displayof elements.

To simply toggle the visibility of an element, meaning its displayis not modified and the element can still affect the flow of the document, use the .invisibleclass instead.