Overview.bs

<pre class='metadata'>
Title: CSS Scrollbars Styling Module Level 1
Group: csswg
Shortname: css-scrollbars
Level: 1
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-scrollbars/
TR: https://www.w3.org/TR/css-scrollbars-1/
Previous version: https://www.w3.org/TR/2021/WD-css-scrollbars-1-20210908/
Previous version: https://www.w3.org/TR/2021/WD-css-scrollbars-1-20210829/
Previous version: https://www.w3.org/TR/2021/WD-css-scrollbars-1-20210805/
Previous version: https://www.w3.org/TR/2018/WD-css-scrollbars-1-20180925/
Repository: https://github.com/w3c/csswg-drafts/tree/master/css-scrollbars-1 css-scrollbars
Editor: Tantek Çelik, Mozilla https://www.mozilla.org/, http://tantek.com/contact, w3cid 1464
Editor: Rossen Atanassov, Microsoft, ratan@microsoft.com, w3cid 49885
Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net/, w3cid 43241
Abstract: This CSS module defines properties
          to influence the visual styling of scrollbars,
          introducing controls for their color and width.
Link Defaults: css-color-3 (property) color
</pre>

<h2 id="intro">
Introduction</h2>

<em>This section is non-normative.</em>

This CSS module introduces properties
to influence the visual styling of scrollbars,
including their color ('scrollbar-color')
and thickness ('scrollbar-width').

<h3 id="scope">
Scope</h3>

The CSS Scrollbars Module is specifically for styling scrollbar controls themselves,
e.g. their color &amp; width in Level 1, and not their layout nor whether any content is scrollable.
All layout impacts and content scrollability are specified in the
<a href="https://drafts.csswg.org/css-overflow/">CSS Overflow Module</a>.

Based on
<a href="https://www.w3.org/wiki/Css-scrollbars#Use-cases">documented use-cases</a>,
there are three main use-cases around scrollbars this module intends to resolve:
<ol>
 <li>Coloring scrollbars to fit better into the UI of a web application.
 <li>Using a thinner scrollbar when the scrolling area is small.
 <li>Hiding UA-provided scrollbars,
  to allow the provision of custom interfaces for scrolling
  without affecting other aspects of scrollability.
</ol>

<h4 id="out-of-scope">
Out Of Scope</h4>

The internal structure, layout, and configuration of scrollbars,
as well as precise control over their coloring, is out of scope.
This is because different platforms have
different scrollbar structures and styling conventions,
and operating systems continuously evolve their scrollbar designs
to provide better user experience.
Pseudo-elements for selecting specific parts of a scrollbar,
for example,
were considered and rejected.
While this level of fine control would be tempting for authors,
the arrangement of the various parts--
or whether they're even all present--
cannot be depended on.
Providing too much control would allow authors to get perfect results on some platforms,
but at the expense of broken results on others.

Note: Exposing the scrollbar-related <code>::-webkit-</code> prefixed pseudo-elements to the Web
is considered a mistake by both the CSS Working Group and Webkit.

<h3 id="values">
Value Definitions</h3>

  This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
  using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
  Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].
  Combination with other CSS modules may expand the definitions of these value types.

  In addition to the property-specific values listed in their definitions,
  all properties defined in this specification
  also accept the <a>CSS-wide keywords</a> as their property value.
  For readability they have not been repeated explicitly.

<h2 id="scrollbar-color">Scrollbar Colors: the 'scrollbar-color' property</h2>

<pre class="propdef">
Name: scrollbar-color
Value: auto | <<color>>{2}
Initial: auto
Inherited: yes
Computed value: specified keyword or two computed colors
Animation type: by computed value
Applies to: [=scroll containers=]
Percentages: n/a
</pre>

This property allows the author to set colors of an element's scrollbars.

UAs must apply the scrollbar-color value set on the root element to the viewport.

Note: Unlike 'overflow' (and overflow-*) properties, 
scrollbar-color value set on the HTML body element are not propagated to the viewport.

<dl dfn-type="value" dfn-for="scrollbar-color">
<dt><dfn>auto</dfn></dt>
<dd>The user agent determines the colors of the scrollbar.
It should follow platform conventions,
but may adjust the colors in accordance with 'color-scheme'
or other contextual information
to better suit the page.
</dd>
<dt><dfn><<color>></dfn></dt>
<dd>
apply the first color to the thumb of the scrollbar,
and the second color to the track of the scrollbar.
</dd>
</dl>

Details:

Track refers to the background of the scrollbar,
which is generally fixed regardless of the scrolling position.

Thumb refers to the moving part of the scrollbar,
which usually floats on top of the track.

If this property computes to a value other than ''scrollbar-color/auto'',
implementations may render a simpler scrollbar than the default platform UI rendering,
and color it accordingly.

Note: Sometimes the UA is unable to customize the colors of native scrollbars,
perhaps due to how they're structured,
or to a lack of control given by the native toolkit.
The provision above allows the UA to replace them with differently-constructed scrollbars,
which it does know how to color.

(Note: add diagram showing the different named pieces - something like 
<a href="http://www.howtocreate.co.uk/tutorials/scrlbar.html">http://www.howtocreate.co.uk/tutorials/scrlbar.html</a>)

(Note: add example of an overflow element with colorized scrollbars to match page styling, 
PNG of the same in a browser that supports it currently)

Implementations may ignore any of the colors
if the corresponding part do not exist on the underlying platform.

When using 'scrollbar-color' property with specific color values,
authors should ensure the specified colors have enough contrast between them.
For keyword values, UAs should ensure the colors they use have enough contrast.
See 
<a href="https://www.w3.org/TR/WCAG21/#non-text-contrast">WCAG 2.1 SC 1.4.11 Non-text Contrast</a> 
[[WCAG21]]. 
UAs may ignore these contrast requirements based on explicit user preferences 
(for example, when users choose a configuration option/setting that 
always ensures a particular scrollbar color / use of system default scrollbars).

Note: when a user interacts with a scrollbar (e.g. hovering or activating), 
implementations may alter which scrollbar colors apply to which scrollbar parts.

Note: IE uses named System Colors as defaults for each of the scrollbar color properties. 
See related <a href="https://github.com/w3c/csswg-drafts/issues/1956">Issue 1956</a>.

  <div class="example">
    <p>The following example 
    (derived from
    <a href="https://www.w3.org/Style/Examples/007/scrollbars.en.html">https://www.w3.org/Style/Examples/007/scrollbars.en.html</a>) 
    resets scrollbar colors in IE.</p>

    <pre><code class="css">
<!-- -->html { 
<!-- -->    scrollbar-color: ThreeDFace Scrollbar;
<!-- -->}</code></pre>
  </div>


<h2 id="scrollbar-width">Scrollbar Thickness: the 'scrollbar-width' property</h2>

<pre class="propdef"> 
Name: scrollbar-width 
Value: auto | thin | none
Initial: auto
Inherited: no
Computed value: specified keyword
Animation type: by computed value
Applies to: [=scroll containers=]
Percentages: n/a
</pre>

This property allows the author to specify
the desired thickness of an element’s scrollbars.

<div class=advisement>
	The primary purpose of this property is not
	to allow authors to chose a particular scrollbar aesthetic for their pages,
	but to let them indicate for certain small or cramped elements of their pages
	that a smaller scrollbar would be desirable.

	Scrollbars are a UI mechanism essential to interact with the page.
	Operating systems tend to want consistency in such controls to improve usability through familiarity,
	and users with specific preferences or needs can adjust
	the appearance of various UI components, including scrollbars,
	through OS or UA settings.

	While using this property in support of specific UX goals is appropriate,
	authors should otherwise refrain from overriding such user preferences.
</div>

<dl dfn-type="value" dfn-for=scrollbar-width>
<dt><dfn>auto</dfn></dt>
<dd>Implementations must use the default scrollbar width.

Note: On most systems, this corresponds to the traditional somewhat wide scrollbar.
However, through OS or UA settings,
users can have the ability to change what this default corresponds to,
possibly making the default scrollbar
wider or narrower than is typical.

</dd>
<dt><dfn>thin</dfn></dt>
<dd>Implementations should use thinner scrollbars than ''scrollbar-width/auto''.
This may mean a thin variant of scrollbar provided by the platform,
or a custom scrollbar thinner than the default platform scrollbar.
The scrollbar must nonetheless remain wide enough to be usable.
(Implementers may wish to consult
<a href="https://www.w3.org/TR/WCAG21/#target-size">WCAG 2.1 SC 2.5.5 Target Size</a>. [[WCAG21]])

Note: User agents can use various strategies
to ensure the usability of narrow scrollbars.
For instance, in the case of <a spec=css-overflow-3>overlay scrollbars</a>,
they can dynamically enlarge the scrollbar
in response to a user attempting to interact with it.
User agents on devices with touch screens
can also adjust how they interpret finger taps
to facilitate interacting with visually small touch targets.

User agents may disregard this value and treat it as ''scrollbar-width/auto'',
for instance when the user has indicated discomfort
for thin scrollbars through some UA or OS setting.
(User agents are encouraged to provide such a setting.)

Note: Some platforms only have a tiny scrollbar by default
which cannot be reasonably made thinner.
In such cases, this value will behave as ''scrollbar-width/auto''.
</dd>
<dt><dfn>none</dfn></dt>
<dd>Implementations must not display any scrollbar,
however the element's scrollability by other means is not affected.

<div class=advisement>
Using this value can prevent mouse-only users from being able to scroll.
Authors should ensure that mouse-only users can still reach hidden content,
even if they have no scrollwheel.
</div>

<div class=advisement>
Authors that use ''scrollbar-width/none'' should provide an alternative/equivalent visual hint that
scrolling is possible and there is more content.
</div>

Note: For situations where an element is to be scrolled <em>only</em> by programmatic means,
and not by direct user manipulation,
authors should use ''overflow: hidden'' instead of ''scrollbar-width: none''.

</dd>
</dl>

<div class=note>Note: Users who find the ''thin'' style of scrollbars unusable
can include the following rule in their [=user style sheet=]:
<pre><code class=lang-css>* { scrollbar-width: auto !important; }</code></pre>
This will ensure that all scrollbars are sized as per OS and UA settings
regardless of author styles.
</div>

UAs must apply the 'scrollbar-width' value set on the root element to the viewport.

Note: Unlike the 'overflow' property (and its longhands),
a 'scrollbar-width' value set on the HTML <{body}> element is not propagated to the viewport.

Note: This specification does not define the exact position or shape of the scrollbar, 
or any animation thereof, such as fading or sliding in/out of view.

<h2 class="no-num" id="acknowledgments">Appendix A. Acknowledgments</h2>

This appendix is <em>informative</em>.

<p>
Thanks to the use-cases, prototyping, implementation, and feedback from
<a class="h-card" href="https://xanthir.com/">Tab Atkins</a> and
<a class="h-card" href="https://www.upsuper.org/">Xidorn Quan</a>.
Thanks to accessibility review and contributions 
(<a href="https://github.com/w3c/csswg-drafts/issues/3315">#3315</a>) 
from 
<a class="h-card" href="https://www.splintered.co.uk">Patrick H. Lauke</a>.
</p>

<h2 class="no-num" id="changes">Appendix B. Changes</h2>

This appendix is <em>informative</em>.

<h3 class="no-num" id="changes-since-2021-08-05">
Changes from the <a href="https://www.w3.org/TR/2021/WD-css-scrollbars-1-20210805/">2021-08-05 Working Draft</a></h3>

	* Switched "should" to a "must" with regards to accessibility of narrow scrollbars.
		(see <a href="https://github.com/w3c/csswg-drafts/issues/6675">Issue 6675</a>)

<h3 class="no-num" id="changes-since-2018-09-25">
Changes from the <a href="https://www.w3.org/TR/2018/WD-css-scrollbars-1-20180925/">2018-09-25 First Public Working Draft</a></h3>

<ul>
<li><a href="https://github.com/w3c/csswg-drafts/issues/6438">#6538</a>:
    removed <css>light</css> and <css>dark</css> values of 'scrollbar-color'
    in favor of allowing the UA to tune ''scrollbar-color/auto''
    in accordance with 'color-scheme' or other contextual information.
<li><a href="https://github.com/w3c/csswg-drafts/issues/3237">#3237</a>: 
    scrollbar-color computed value changed to: specified keyword or two computed colors
</li>
<li><a href="https://github.com/w3c/csswg-drafts/issues/4693">#4693</a>: 
    Clarified scope: styling scrollbar controls themselves, no layout or scrollability.
</li>
<li><a href="https://github.com/w3c/csswg-drafts/issues/3315">#3315</a>: 
    More and updated accessibility considerations for scrollbar-color and scrollbar-width.
</ul>

<h2 class="no-num" id="security-privacy-considerations">Appendix C. Considerations for Security and Privacy</h2>

This appendix is <em>informative</em>.

<h3 class="no-num" id="security-considerations">Considerations for Security</h3>

	No specific concerns regarding security have been identified for this specification.

<h3 class="no-num" id="privacy-considerations">Considerations for Privacy</h3>

	No specific concerns regarding privacy have been identified for this specification.

<h3 class="no-num" id="security-privacy-self-review">Self-review questionaire</h3>

Per the <a href="https://www.w3.org/TR/security-privacy-questionnaire/#questions">
Self-Review Questionnaire: Security and Privacy: Questions to Consider</a>

<ol>
<li>Does this specification deal with personally-identifiable information?
<p>No.</p>
</li>

<li>Does this specification deal with high-value data?
<p>No.</p>
</li>

<li>Does this specification introduce new state for an origin that persists across browsing sessions?
<p>No.</p>
</li>

<li>Does this specification expose persistent, cross-origin state to the web?
<p>No.</p>
</li>

<li>Does this specification expose any other data to an origin that it doesn’t currently have access to?
<p>No.</p>
</li>

<li>Does this specification enable new script execution/loading mechanisms?
<p>No.</p>
</li>

<li>Does this specification allow an origin access to a user’s location?
<p>No.</p>
</li>

<li>Does this specification allow an origin access to sensors on a user’s device?
<p>No.</p>
</li>

<li>Does this specification allow an origin access to aspects of a user’s local computing environment?
<p>No.</p>
</li>

<li>Does this specification allow an origin access to other devices?
<p>No.</p>
</li>

<li>Does this specification allow an origin some measure of control over a user agent’s native UI?
<p>Yes. The 'scrollbar-*' properties enable the page to change the color and width of the scrollbar 
of the user agent’s native UI, e.g. scrollbars on the page’s window, on framed content embedded in the page, 
or on overflowing elements with scrollbars in the page.</p>
</li>

<li>Does this specification expose temporary identifiers to the web?
<p>No.</p>
</li>

<li>Does this specification distinguish between behavior in first-party and third-party contexts?
<p>No.</p>
</li>

<li>How should this specification work in the context of a user agent’s "incognito" mode?
<p>No differently.</p>
</li>

<li>Does this specification persist data to a user’s local device?
<p>No.</p>
</li>

<li>Does this specification have a "Security Considerations" and "Privacy Considerations" section?
<p>Yes.</p>
</li>

<li>Does this specification allow downgrading default security characteristics?
<p>No.</p>
</li>
</ol>

<h2 class="no-num" id="accessibility-considerations">Appendix D. Considerations for accessibility</h2>

This appendix is <em>informative</em>.

As noted [[#scrollbar-width|in the definition of the property]],
authors need to be mindful of the accessibility implications
of using ''scrollbar-width: thin''.
Scrollbars are a important piece of the user agent's interface,
and it is not appropriate for a web site author to change their size over aesthetic considerations.
The property is available to support cases
where the author wants to indicate that in a cramped area of the web page
a thin scrollbar would be a more effective use of space.
However, ultimately, the user, through their user agent,
needs to have the last word on such things.

Using this property in such cases is preferable to
authors building a custom thin-looking scrollbar in via script or proprietary extensions,
because it does give the user the opportunity to override it.

[=User style sheets=] do provide such an override,
and additionally, user agents are encouraged to expose a setting
letting users express that they do not want thin scrollbars to be used.

The CSS Working Group also acknowledges the needs of some users
to have scrollbars that are wider than is typical.
Operating systems and user agents can offer a means to let users express that preference,
and in such cases, CSS will honor that choice.