Bootstrap/reboot.md at 723a03cc43f43aabf81e10e800ce6d1b14207a77

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layout	title	description	group	redirect_from	toc
docs	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.	content	/content/	true

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.

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.
The <body> also sets a global font-family and line-height. 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.

{% highlight sass %} $font-family-sans-serif: // Safari for OS X and iOS (San Francisco) -apple-system, // Chrome >= 56 for OS X (San Francisco), Windows, Linux and Android system-ui, // Chrome < 56 for OS X (San Francisco) BlinkMacSystemFont, // Windows "Segoe UI", // Android "Roboto", // Basic web fallback "Helvetica Neue", Arial, sans-serif !default; {% endhighlight %}

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.

Heading	Example
{% markdown %}` `{% endmarkdown %}	h1. Bootstrap heading
{% markdown %}` `{% endmarkdown %}	h2. Bootstrap heading
{% markdown %}` `{% endmarkdown %}	h3. Bootstrap heading
{% markdown %}` `{% endmarkdown %}	h4. Bootstrap heading
{% markdown %}` `{% endmarkdown %}	h5. Bootstrap heading
{% markdown %}` `{% endmarkdown %}	h6. Bootstrap heading

Lists

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

{% markdown %} * Lorem ipsum dolor sit amet * Consectetur adipiscing elit * 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 elit
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 {% endmarkdown %}

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.

{% markdown %}

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.

{% endmarkdown %}

Preformatted text

The <pre> element is reset to remove its margin-top and use rem units for its margin-bottom.

{% markdown %}

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

{% endmarkdown %}

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]({{ site.baseurl }}/docs/{{ site.docs_version }}/content/tables/).

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-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.

These changes, and more, are demonstrated below.

Example legend

<p>
  <label for="input">Example input</label>
  <input type="text" id="input" placeholder="Example input">
</p>

<p>
  <label for="select">Example select</label>
  <select id="select">
    <option value="">Choose...</option>
    <optgroup label="Option group 1">
      <option value="">Option 1</option>
      <option value="">Option 2</option>
      <option value="">Option 3</option>
    </optgroup>
    <optgroup label="Option group 2">
      <option value="">Option 4</option>
      <option value="">Option 5</option>
      <option value="">Option 6</option>
    </optgroup>
  </select>
</p>

<p>
  <label>
    <input type="checkbox" value="">
    Check this checkbox
  </label>
</p>

<p>
  <label>
    <input type="radio" name="optionsRadios" id="optionsRadios1" value="option1" checked>
    Option one is this and that
  </label>
  <label>
    <input type="radio" name="optionsRadios" id="optionsRadios2" value="option2">
    Option two is something else that's also super long to demonstrate the wrapping of these fancy form controls.
  </label>
  <label>
    <input type="radio" name="optionsRadios" id="optionsRadios3" value="option3" disabled>
    Option three is disabled
  </label>
</p>

<p>
  <label for="textarea">Example textarea</label>
  <textarea id="textarea" rows="3"></textarea>
</p>

<p>
  <label for="time">Example temporal</label>
  <input type="datetime-local" id="time">
</p>

<p>
  <label for="output">Example output</label>
  <output name="result" id="output">100</output>
</p>

<p>
  <button type="submit">Button submit</button>
  <input type="submit" value="Input submit button">
  <input type="button" value="Input button">
</p>

<p>
  <button type="submit" disabled>Button submit</button>
  <input type="submit" value="Input submit button" disabled>
  <input type="button" value="Input button" disabled>
</p>

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>.

Twitter, Inc.
1355 Market St, Suite 900
San Francisco, CA 94103
P: (123) 456-7890 Full Name
first.last@example.com

Blockquote

The default margin on blockquotes is 1em 40px, so we reset that to 0 0 1rem for 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 amongst paragraph text.

Nulla attr vitae elit libero, a pharetra augue.

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. While [hidden] isn't natively supported by IE10, the explicit declaration in our CSS gets around that problem.

{% highlight html %} {% endhighlight %}

{% callout warning %}

jQuery incompatibility

[hidden] is not compatible with jQuery's $(...).hide() and $(...).show() methods. This could potentially change in jQuery 3, but we're not holding our breath. Therefore, we don't currently especially endorse [hidden] over other techniques for managing the display of elements. {% endcallout %}

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]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/invisible-content/) instead.

Click delay optimization for touch

Traditionally, browsers on touchscreen devices have a delay of approximately 300ms between the end of a "tap" – the moment when a finger/stylus is lifted from screen – and the click event being fired. This delay is necessary for these browsers to correctly handle "double-tap to zoom" gestures without prematurely triggering actions or links after the first "tap", but it can make your site feel slightly sluggish and unresponsive.

Most mobile browsers automatically optimize away this 300ms delay for sites that use the width=device-width property as part of their [responsive meta tag]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/introduction/#responsive-meta-tag) (as well as for sites that disable zooming, for instance with user-scalable=no, though this practice is strongly discouraged for accessibility and usability reasons). The biggest exceptions here are IE11 on Windows Phone 8.1, and iOS Safari (and any other iOS WebView-based browser) prior to iOS 9.3.

On touch-enabled laptop/desktop devices, IE11 and Microsoft Edge are currently the only browsers with "double-tap to zoom" functionality. As the [responsive meta tag]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/introduction/#responsive-meta-tag) is ignored by all desktop browsers, using width=device-width will have no effect on the 300ms delay here.

To address this problem in IE11 and Microsoft Edge on desktop, as well as IE11 on Windows Phone 8.1, Bootstrap explicitly uses the touch-action:manipulation CSS property on all interactive elements (such as buttons and links). This property essentially disables double-tap functionality on those elements, eliminating the 300ms delay.

In the case of old iOS versions (prior to 9.3), the suggested approach is to use additional scripts such as FastClick to explicitly work around the delay.

For further details, see the compatibility table for suppressing 300ms delay for touchscreen interactions.

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