Overview.bs

<pre class='metadata'>
Title: CSS Scrollbars Module Level 1
Group: csswg
Shortname: css-scrollbars
Level: 1
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-scrollbars/
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, width, and impact on layout.
Link Defaults: css-color-3 (property) color
</pre>

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

Note: this specification is 
<a href="https://github.com/w3c/csswg-drafts/tree/master/css-scrollbars-1">
maintained in the W3C csswg-drafts GitHub repository as css-scrollbars-1
</a>.

<h3 id="scope">
Scope</h3>
<em>This subsection is non-normative.</em>

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>Building customized scrollbars without affecting scrollability.
</ol>

For addressing these use-cases, this module adds properties to control the colors and
width of scrollbars (if any) of an element.

<h4 id="out-of-scope">
Out Of Scope</h4>
Layout of scrollbars and related or adjacent content, and whether content is scrollable, is out of scope.
Those features are specified in the 
<a href="https://drafts.csswg.org/css-overflow/">CSS Overflow Module</a>.

Pseudo-elements for selecting specific parts of a scrollbar are out of scope.
The WebKit implementation of pseudo-elements for scrollbar is
considered to be a feature mistakenly exposed to the web.
Main concerns against the pseudo-elements approach are:
<ul>
 <li>Operating systems continuously evolve their scrollbar designs to provide better user experience,
 beyond the ability of any set of pseudo-elements to accurately model this over time.
 <li>Different platforms have different scrollbar structure means testing interop is harder,
 because authors would need to take not only engine but also platforms into account.
</ul>

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

(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.

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

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>.

This appendix describes changes from the
<a href="https://www.w3.org/TR/2018/WD-css-scrollbars-1-20180925/">First Public Working Draft (FPWD) of 25 Sep 2018</a>.
<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.
</li>
</ul>

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

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

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>