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---
layout: docs
title: Reboot
description: 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.
group: content
aliases: "/docs/5.3/content/"
toc: 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 `
` 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 `rem`s instead of `em`s 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 `rem`s for `margin`s.
- Keep declarations of `font`-related properties to a minimum, using `inherit` whenever possible.
## CSS variables
{{< added-in "5.2.0" >}}
With v5.1.1, we standardized our required `@import`s 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]({{< docsref "/customize/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 `` styles:
{{< scss-docs name="root-body-variables" file="scss/_root.scss" >}}
In practice, those variables are then applied in Reboot like so:
{{< scss-docs name="reboot-body-rules" file="scss/_reboot.scss" >}}
Which allows you to make real-time customizations however you like:
```html
```
## Page defaults
The `` and `` 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 ``, but `16px` is assumed (the browser default). `font-size: 1rem` is applied on the `` 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 `` 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 `` 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](https://www.smashingmagazine.com/2015/11/using-system-ui-fonts-practical-guide/).
```scss
$font-family-sans-serif:
// Cross-platform generic font family (default user interface font)
system-ui,
// Safari for macOS and iOS (San Francisco)
-apple-system,
// Windows
"Segoe UI",
// Android
Roboto,
// older macOS and iOS
"Helvetica Neue"
// Linux
"Noto Sans",
"Liberation Sans",
// Basic web fallback
Arial,
// Sans serif fallback
sans-serif,
// Emoji fonts
"Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji" !default;
```
Note that because the font stack includes emoji fonts, many common symbol/dingbat Unicode characters will be rendered as multicolored pictographs. Their appearance will vary, depending on the style used in the browser/platform's native emoji font, and they won't be affected by any CSS `color` styles.
This `font-family` is applied to the `` and automatically inherited globally throughout Bootstrap. To switch the global `font-family`, update `$font-family-base` and recompile Bootstrap.
## Headings
All heading elements—``—`` have their `margin-top` removed, `margin-bottom: .5rem` set, and `line-height` tightened. While headings inherit their `color` by default, you can also override it via optional CSS variable, `--bs-heading-color`.
{{< bs-table "table" >}}
| Heading | Example |
| --- | --- |
| `` | h1. Bootstrap heading |
| `` | h2. Bootstrap heading |
| `` | h3. Bootstrap heading |
| `` | h4. Bootstrap heading |
| `` | h5. Bootstrap heading |
| `` | h6. Bootstrap heading |
{{< /bs-table >}}
## Paragraphs
All `
` elements have their `margin-top` removed and `margin-bottom: 1rem` set for easy spacing.
{{< example >}}
This is an example paragraph.
{{< /example >}}
## Links
Links have a default `color` and underline applied. While links change on `:hover`, they don't change based on whether someone `:visited` the link. They also receive no special `:focus` styles.
{{< example >}}
This is an example link
{{< /example >}}
As of v5.3.x, link `color` is set using `rgba()` and new `-rgb` CSS variables, allowing for easy customization of link color opacity. Change the link color opacity with the `--bs-link-opacity` CSS variable:
{{< example >}}
This is an example link
{{< /example >}}
Placeholder links—those without an `href`—are targeted with a more specific selector and have their `color` and `text-decoration` reset to their default values.
{{< example >}}
This is a placeholder link
{{< /example >}}
## Horizontal rules
The `
` element has been simplified. Similar to browser defaults, `
`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.
{{< example >}}
{{< /example >}}
## Lists
All lists—``, ``, and ``—have their `margin-top` removed and a `margin-bottom: 1rem`. Nested lists have no `margin-bottom`. We've also reset the `padding-left` on `