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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 margin is 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 inherit whenever possible.

CSS variables

Added in v5.2.0

With v5.1.1, we standardized our required @imports across all our CSS bundles (including bootstrap.css, bootstrap-reboot.css, and bootstrap-grid.css) to include _root.scss. This adds :root level CSS variables to all bundles, regardless of how many of them are used in that bundle. Ultimately Bootstrap 5 will continue to see more CSS variables added over time, in order to provide more real-time customization without the need to always recompile Sass. Our approach is to take our source Sass variables and transform them into CSS variables. That way, even if you don’t use CSS variables, you still have all the power of Sass. This is still in-progress and will take time to fully implement.

For example, consider these :root CSS variables for common <body> styles:

  1.   @if $font-size-root != null {
  2.     --#{$prefix}root-font-size: #{$font-size-root};
  3.   }
  4.   --#{$prefix}body-font-family: #{$font-family-base};
  5.   @include rfs($font-size-base, --#{$prefix}body-font-size);
  6.   --#{$prefix}body-font-weight: #{$font-weight-base};
  7.   --#{$prefix}body-line-height: #{$line-height-base};
  8.   --#{$prefix}body-color: #{$body-color};
  9.   @if $body-text-align != null {
  10.     --#{$prefix}body-text-align: #{$body-text-align};
  11.   }
  12.   --#{$prefix}body-bg: #{$body-bg};

In practice, those variables are then applied in Reboot like so:

  1. body {
  2.   margin: 0; // 1
  3.   font-family: var(--#{$prefix}body-font-family);
  4.   @include font-size(var(--#{$prefix}body-font-size));
  5.   font-weight: var(--#{$prefix}body-font-weight);
  6.   line-height: var(--#{$prefix}body-line-height);
  7.   color: var(--#{$prefix}body-color);
  8.   text-align: var(--#{$prefix}body-text-align);
  9.   background-color: var(--#{$prefix}body-bg); // 2
  10.   -webkit-text-size-adjust: 100%; // 3
  11.   -webkit-tap-highlight-color: rgba($black, 0); // 4
  12. }

Which allows you to make real-time customizations however you like:

  1. <body style="--bs-body-color: #333;">
  2.   <!-- ... -->
  3. </body>

Page defaults

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

  • The box-sizing is globally set on every element—including *::before and *::after, to border-box. This ensures that the declared width of element is never exceeded due to padding or border.
    • No base font-size is declared on the <html>, but 16px is assumed (the browser default). font-size: 1rem is applied on the <body> for easy responsive type-scaling via media queries while respecting user preferences and ensuring a more accessible approach. This browser default can be overridden by modifying the $font-size-root variable.
  • The <body> also sets a global font-family, font-weight, line-height, and color. 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

Bootstrap utilizes a “native font stack” or “system font stack” for optimum text rendering on every device and OS. These system fonts have been designed specifically with today’s devices in mind, with improved rendering on screens, variable font support, and more. Read more about native font stacks in this Smashing Magazine article.

  1. $font-family-sans-serif:
  2.   // Cross-platform generic font family (default user interface font)
  3.   system-ui,
  4.   // Safari for macOS and iOS (San Francisco)
  5.   -apple-system,
  6.   // Windows
  7.   "Segoe UI",
  8.   // Android
  9.   Roboto,
  10.   // older macOS and iOS
  11.   "Helvetica Neue"
  12.   // Linux
  13.   "Noto Sans",
  14.   "Liberation Sans",
  15.   // Basic web fallback
  16.   Arial,
  17.   // Sans serif fallback
  18.   sans-serif,
  19.   // Emoji fonts
  20.   "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji" !default;

Note that because the font stack includes emoji fonts, many common symbol/dingbat Unicode characters will be rendered as multicolored pictographs. Their appearance will vary, depending on the style used in the browser/platform’s native emoji font, and they won’t be affected by any CSS color styles.

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

Headings and paragraphs

All heading elements—e.g., <h1>—and <p> are reset to have their margin-top removed. Headings have margin-bottom: .5rem added and paragraphs margin-bottom: 1rem for easy spacing.

HeadingExample
<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

Horizontal rules

The <hr> element has been simplified. Similar to browser defaults, <hr>s are styled via border-top, have a default opacity: .25, and automatically inherit their border-color via color, including when color is set via the parent. They can be modified with text, border, and opacity utilities.

Reboot - 图1

html

  1. <hr>
  3. <div class="text-success">
  4.   <hr>
  5. </div>
  7. <hr class="border border-danger border-2 opacity-50">
  8. <hr class="border border-primary border-3 opacity-75">

Lists

All lists—<ul>, <ol>, and <dl>—have their margin-top removed and a margin-bottom: 1rem. Nested lists have no margin-bottom. We’ve also reset the padding-left on <ul> and <ol> elements.

Reboot - 图2

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

Reboot - 图3

Inline code

Wrap inline snippets of code with <code>. Be sure to escape HTML angle brackets.

Reboot - 图4

html

  1. For example, <code>&lt;section&gt;</code> should be wrapped as inline.

Code blocks

Use <pre>s for multiple lines of code. Once again, be sure to escape any angle brackets in the code for proper rendering. The <pre> element is reset to remove its margin-top and use rem units for its margin-bottom.

Reboot - 图5

html

  1. <pre><code>&lt;p&gt;Sample text here...&lt;/p&gt;
  2. &lt;p&gt;And another line of sample text here...&lt;/p&gt;
  3. </code></pre>

Variables

For indicating variables use the <var> tag.

Reboot - 图6

html

  1. <var>y</var> = <var>m</var><var>x</var> + <var>b</var>

User input

Use the <kbd> to indicate input that is typically entered via keyboard.

Reboot - 图7

html

  1. To switch directories, type <kbd>cd</kbd> followed by the name of the directory.<br>
  2. To edit settings, press <kbd><kbd>ctrl</kbd> + <kbd>,</kbd></kbd>

Sample output

For indicating sample output from a program use the <samp> tag.

Reboot - 图8

html

  1. <samp>This text is meant to be treated as sample output from a computer program.</samp>

Tables

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

Reboot - 图9

html

  1. <table>
  2.   <caption>
  3.     This is an example table, and this is its caption to describe the contents.
  4.   </caption>
  5.   <thead>
  6.     <tr>
  7.       <th>Table heading</th>
  8.       <th>Table heading</th>
  9.       <th>Table heading</th>
  10.       <th>Table heading</th>
  11.     </tr>
  12.   </thead>
  13.   <tbody>
  14.     <tr>
  15.       <td>Table cell</td>
  16.       <td>Table cell</td>
  17.       <td>Table cell</td>
  18.       <td>Table cell</td>
  19.     </tr>
  20.     <tr>
  21.       <td>Table cell</td>
  22.       <td>Table cell</td>
  23.       <td>Table cell</td>
  24.       <td>Table cell</td>
  25.     </tr>
  26.     <tr>
  27.       <td>Table cell</td>
  28.       <td>Table cell</td>
  29.       <td>Table cell</td>
  30.       <td>Table cell</td>
  31.     </tr>
  32.   </tbody>
  33. </table>

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-block to allow margin to be applied.
  • <input>s, <select>s, <textarea>s, and <button>s are mostly addressed by Normalize, but Reboot removes their margin and sets line-height: inherit, too.
  • <textarea>s are modified to only be resizable vertically as horizontal resizing often “breaks” page layout.
  • <button>s and <input> button elements have cursor: pointer when :not(:disabled).

These changes, and more, are demonstrated below.

Reboot - 图10

Date & color input support

Keep in mind date inputs are not fully supported by all browsers, namely Safari.

Pointers on buttons

Reboot includes an enhancement for role="button" to change the default cursor to pointer. Add this attribute to elements to help indicate elements are interactive. This role isn’t necessary for <button> elements, which get their own cursor change.

Reboot - 图11

html

  1. <span role="button" tabindex="0">Non-button element button</span>

Misc elements

Address

The <address> element is updated to reset the browser default font-style from italic to normal. line-height is also now inherited, and margin-bottom: 1rem has 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>.

Reboot - 图12

Blockquote

The default margin on blockquotes is 1em 40px, so we reset that to 0 0 1rem for something more consistent with other elements.

Reboot - 图13

Inline elements

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

Reboot - 图14

Summary

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

Reboot - 图15

HTML5 [hidden] attribute

HTML5 adds a new global attribute named [hidden], which is styled as display: none by default. Borrowing an idea from PureCSS, we improve upon this default by making [hidden] { display: none !important; } to help prevent its display from getting accidentally overridden.

  1. <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 display of elements.

To merely toggle the visibility of an element, meaning its display is not modified and the element can still affect the flow of the document, use the .invisible class instead.