Overview.bs

<pre class="metadata">
Title: CSS Scroll Snap Module Level 1
Group: csswg
Shortname: css-scroll-snap
TR: https://www.w3.org/TR/css-scroll-snap-1/
Level: 1
Status: ED
Previous Version: https://www.w3.org/TR/2017/CR-css-scroll-snap-1-20170824/
Previous Version: https://www.w3.org/TR/2017/CR-css-scroll-snap-1-20170209/
Previous Version: https://www.w3.org/TR/2016/CR-css-scroll-snap-1-20161020/
Previous Version: https://www.w3.org/TR/2016/WD-css-scroll-snap-1-20160623/
Previous Version: https://www.w3.org/TR/2016/WD-css-snappoints-1-20160329/
Previous Version: https://www.w3.org/TR/2015/WD-css-snappoints-1-20150326/
Work Status: Testing
ED: https://drafts.csswg.org/css-scroll-snap-1/
Editor: Matt Rakow, Microsoft, w3cid 62267
Editor: Jacob Rossi, Microsoft, w3cid 45616
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Abstract: This module contains features to control panning and scrolling behavior with “snap positions”.
Ignored Terms: containing block chain, scroll position, scrollport
At Risk: 'scroll-snap-stop'
Status Text:
 A test suite and an implementation report will be produced during the
 CR period.
</pre>

<pre class=link-defaults>
spec: selectors-4; type: selector; text: :target
spec: css-pseudo-4; type: selector;
    text: ::first-line
    text: ::first-letter
</pre>

Introduction {#intro}
=====================

    <em>This section is not normative.</em>

    Popular UX paradigms for scrollable content frequently employ paging through content,
    or sectioning into logical divisions.
    This is especially true for touch interactions
    where it is quicker and easier for users to quickly pan through a flatly-arranged breadth of content
    rather than delving into a heirarchical structure through tap navigation.
    For example, it is easier for a user to view many photos in a photo album
    by panning through a photo slideshow view
    rather than tapping on individual photos in an album.

    However, given the imprecise nature of scrolling inputs
    like touch panning and mousewheel scrolling,
    it is difficult for web developers to guarantee a well-controlled scrolling experience,
    in particular creating the effect of paging through content.
    For instance, it is easy for a user to land at an awkward scroll position
    which leaves a page partially on-screen when panning.

    To this end, we introduce scroll snap positions
    which enforce the scroll positions that a <a>scroll container’s</a> scrollport may end at
    after a scrolling operation has completed.

Module interactions {#placement}
--------------------------------

    This module extends the scrolling user interface features defined in [[!CSS2]] section 11.1.

    None of the properties in this module apply to the ''::first-line'' and ''::first-letter'' pseudo-elements.

Values {#values}
----------------

    This specification follows the <a href="https://www.w3.org/TR/CSS21/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]].
    Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
    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> keywords as their property value.
    For readability they have not been repeated explicitly.

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Motivating Examples {#examples}
===============================

    <div class="example">
        In this example, a series of images arranged in a <a>scroll container</a>
        are used to build a photo gallery.  In this example the <a>scroll container</a>
        is larger than the photos contained within (such that multiple images may be seen simultaneously), and the image
        sizes vary.  Using mandatory element-based snap
        positions, scrolling will always complete with an image centered in the <a>scroll container’s</a> scrollport.

        <pre class="lang-css">
            img {
                /* Specifies that the center of each photo
                   should align with the center of the scroll
                   container in the X axis when snapping */
                scroll-snap-align: center none;
            }
            .photoGallery {
                width: 500px;
                overflow-x: auto;
                overflow-y: hidden;
                white-space: nowrap;
                /* Requires that the scroll position always be
                   at a snap position when the scrolling
                   operation completes. */
                scroll-snap-type: x mandatory;
            }
        </pre>

        <pre class="lang-html">
            &lt;div class="photoGallery">
                &lt;img src="img1.jpg">
                &lt;img src="img2.jpg">
                &lt;img src="img3.jpg">
                &lt;img src="img4.jpg">
                &lt;img src="img5.jpg">
            &lt;/div>
        </pre>

        <figure>
            <img src="images/element_snap_positions.png" alt="">

            <figcaption>
                The layout of the scroll container’s contents in the example.
                The snapport is represented by the red rectangle, and the snap area is represented by the yellow rectangle.  Since the scroll-snap-align is “center” in the X axis, a snap position is established at each scroll position which aligns the X-center of the snapport (represented by a red dotted line) with the X-center of a snap area (represented by a yellow dotted line).
            </figcaption>
        </figure>
    </div>

    <div class="example">
        This example builds a paginated document that aligns each page near to (but not exactly on) the edge of the <a>scroll container</a>.
        This allows the previous page to “peek” in from above in order to make the user aware that they are not yet at the top of the document.
        Using proximity snap positions instead of mandatory snap positions allows the user to stop halfway through a page (rather than forcing them
        to snap one page at a time).  However, if a scrolling operation would finish near a snap position, then the scroll will be adjusted to
        align the page as specified.

        <pre class="lang-css">
            .page {
                /* Defines the top of each page as the
                   edge that should be used for snapping */
                scroll-snap-align: none start;
            }
            .docScroller {
                width: 500px;
                overflow-x: hidden;
                overflow-y: auto;
                /* Specifies that each element’s snap area should
                   align with a 100px offset from the top edge. */
                scroll-padding: 100px 0 0;
                /* Encourages scrolling to end at a snap position when the
                    operation completes, if it is near a snap position */
                scroll-snap-type: y proximity;
            }
        </pre>

        <pre class="lang-html">
            &lt;div class="docScroller">
                &lt;div class="page">Page 1&lt;/div>
                &lt;div class="page">Page 2&lt;/div>
                &lt;div class="page">Page 3&lt;/div>
                &lt;div class="page">Page 4&lt;/div>
            &lt;/div>
        </pre>

        <figure>
            <img src="images/element_snap_positions_offset.png" alt="">

            <figcaption>
                The layout of the scroll container’s contents in the example.
                The snapport is represented by the red rectangle
                (inset from the top by 100px due to the scroll-padding),
                and the snap area is represented by the yellow rectangle.
                Since the scroll-snap-align is “start” in the Y axis,
                a snap position is established at each scroll position
                which aligns the Y-start of the snapport
                (represented by a red dotted line)
                with the Y-start of a snap area
                (represented by a yellow dotted line).
            </figcaption>
        </figure>
    </div>

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Overview {#overview}
====================

    This module introduces control over <dfn lt="scroll snap position" local-lt="snap position">scroll snap positions</dfn>,
    which are scroll positions that produce particular alignments
    of content within a scroll container.
    Using the 'scroll-snap-type' property on the relevant <a>scroll container</a>,
    the author can request a particular bias
    for the scrollport to land on a <a>snap position</a>
    after scrolling operations.

    <a>Snap positions</a> are specified
    as a particular alignment ('scroll-snap-align')
    of an element’s <a>scroll snap area</a>
    (its border bounding box, as modified by 'scroll-margin')
    within the <a>scroll container</a>’s <a>snapport</a>
    (its scrollport, as reduced by 'scroll-padding').
    This is conceptually equivalent to specifying the alignment of
    an <a>alignment subject</a> within an <a>alignment container</a>.
    A scroll position that satisfies the specified alignment
    is a <a>snap position</a>.

    The act of adjusting the scroll position
    of a scroll container’s scrollport
    such that it is aligned to a snap position
    is called <dfn lt="snap | snapping | snapped">snapping</dfn>,
    and a <a>scroll container</a> is said to be
    <a>snapped</a> to a <a>snap position</a>
    if its scrollport’s scroll position
    is that <a>snap position</a>
    and there is no active scrolling operation.
    The CSS Scroll Snap Module
    intentionally does not specify nor mandate
    any precise animations or physics used to enforce <a>snap positions</a>;
    this is left up to the user agent.

    <a>Snap positions</a> only affect the nearest ancestor <a>scroll container</a>
    on the element’s <a>containing block chain</a>.


Capturing Scroll Snap Areas:  Properties on the scroll container {#properties-on-the-scroll-container}
======================================================================================================

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Scroll Snapping Rules: the 'scroll-snap-type' property {#scroll-snap-type}
--------------------------------------------------------------------------

    <pre class="propdef">
    Name: scroll-snap-type
    Value: none | [ x | y | block | inline | both ] [ mandatory | proximity ]?
    Initial: none
    Applies to: all elements
    Inherited: no
    Percentages: n/a
    Media: interactive
    Computed value: as specified
    Animatable: no
    </pre>

    The 'scroll-snap-type' property specifies
    whether a <a>scroll container</a> is a <a>scroll snap container</a>,
    how <a href="#snap-strictness">strictly</a> it <a>snaps</a>,
    and <a href="#snap-axis">which axes</a> are considered.
    If no strictness value is specified, ''proximity'' is assumed.

    <div class="example">
      In this example, snapping to headings is enabled in the <a>block axis</a>
      (the y axis for horizontal writing, x axis for vertical writing):

      <pre>
        html {
          scroll-snap-type: block;   /* applied to main document scroller */
        }
        h1, h2, h3, h4, h5, h6 {
          scroll-snap-align: start;  /* snap to the start (top) of the viewport */
        }
      </pre>
    </div>

<h4 id="snap-axis">
Scroll Snap Axis: the ''x'', ''y'', ''scroll-snap-type/block'', ''scroll-snap-type/inline'', and ''both'' values</h4>

    The <dfn noexport lt="axis value">axis values</dfn>
    specify what axis(es) are affected by <a>snap positions</a>,
    and whether <a>snap positions</a> are evaluated independently per axis,
    or together as a 2D point.
    Values are defined as follows:

    <dl dfn-type=value dfn-for="scroll-snap-type">
        <dt><dfn>x</dfn>
        <dd>
            The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
            in its horizontal axis only.

        <dt><dfn>y</dfn>
        <dd>
            The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
            in its vertical axis only.

        <dt><dfn>block</dfn>
        <dd>
            The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
            in its block axis only.

        <dt><dfn>inline</dfn>
        <dd>
            The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
            in its inline axis only.

        <dt><dfn>both</dfn>
        <dd>
            The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
            in both of its axes independently
            (potentially snapping to different elements in each axis).
    </dl>

<h4 id="snap-strictness">
Scroll Snap Strictness: the ''scroll-snap-type/none'', ''proximity'', and ''mandatory'' values</h4>

    The <dfn noexport lt="strictness value">strictness values</dfn>
    (''scroll-snap-type/none'', ''proximity'', ''mandatory'')
    specify how strictly
    <a>snap positions</a> are enforced on the <a>scroll container</a>
    (by forcing an adjustment to the scroll position).
    Values are defined as follows:

    <dl dfn-type="value" dfn-for="scroll-snap-type">
        <dt><dfn>none</dfn>
        <dd>
            If specified on a <a>scroll container</a>,
            the <a>scroll container</a> must not <a>snap</a>.

        <dt><dfn>mandatory</dfn>
        <dd>
            If specified on a <a>scroll container</a>,
            the <a>scroll container</a> is required to be <a>snapped</a> to a snap position
            when there are no active scrolling operations.
            If a reachable snap position exists
            then the scroll container must <a>snap</a> at the termination of a scroll
            (if none exist then no <a>snapping</a> occurs).


        <dt><dfn>proximity</dfn>
        <dd>
            If specified on a <a>scroll container</a>,
            the <a>scroll container</a> may <a>snap</a> to a snap position
            at the termination of a scroll,
            at the discretion of the UA given the parameters of the scroll.
    </dl>

    Advisement:
    Authors should use mandatory snap positions with consideration of
    varyingly-sized screens and (if applicable) varying-sized content.
    In particular, although access to snapped elements larger than the scrollport
    is handled by the UA,
    if authors assign mandatory snapping to non-adjacent siblings,
    content in between can become inaccessible
    in cases where it is longer than the screen.

    A box <dfn export>captures snap positions</dfn>
    if it is a <a>scroll container</a>
    <em>or</em> has a value other than ''scroll-snap-type/none'' for 'scroll-snap-type'.
    If a box’s nearest <a lt="captures snap positions">snap-position capturing</a> ancestor
    on its <a>containing block chain</a>
    is a <a>scroll container</a> with a non-''scroll-snap-type/none'' value for 'scroll-snap-type',
    that is the box’s <dfn export local-lt="snap container">scroll snap container</dfn>.
    Otherwise, the box has no <a>scroll snap container</a>,
    and its <a>snap positions</a> do not trigger <a>snapping</a>.

    If the content or layout of the document changes
    (e.g. content is added, moved, deleted, resized)
    such that the content of a <a>snapport</a> changes,
    the UA must re-evaluate the resulting <a>scroll position</a>,
    and re-snap if required.
    If the <a>scroll container</a> was <a>snapped</a> before the content change
    and that same <a>snap position</a> still exists
    (e.g. its associated element was not deleted),
    the scroll container must be re-snapped to that same snap position
    after the content change.

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Scroll Snapport: the 'scroll-padding' property {#scroll-padding}
----------------------------------------------------------------

    <pre class="propdef">
    Name: scroll-padding
    Value: <<length-percentage>>{1,4}
    Initial: 0
    Applies to: <a>scroll containers</a>
    Inherited: no
    Percentages: relative to the corresponding dimension of the scroll container’s scrollport
    Media: interactive
    Computed value: as specified, with lengths made absolute
    Animatable: as length, percentage, or calc
    </pre>

    This property is a <a>shorthand property</a> that sets
    all of the <a href="#longhands"><css>scroll-padding-*</css> longhands</a>
    in one declaration,
    assigning values to the longhands representing each side
    exactly as the 'padding' property does for its longhands.

    Values must be non-negative
    and represent inward offsets from the corresponding edge of the scrollport.
    These offsets define the
    <dfn export>optimal viewing region</dfn> of the scrollport:
    the region used as the target region for placing things in view of the user.
    This allows the author to exclude regions of the scrollport
    that are obscured by other content
    (such as fixed-positioned toolbars or sidebars)
    or simply to put more breathing room
    between a targetted element and the edges of the scrollport.

    The 'scroll-padding' offsets effectively reduce the region of the viewport
    that is considered “viewable” <em>for scrolling operations</em>:
    they have no effect on layout,
    on the scroll origin or initial position,
    or on whether or not an element is considered actually <em>visible</em>,
    but should
    affect whether an element or the caret is considered scrolled into view,
    and reduce the amount of scrolling for paging operations
    (such as using the <kbd>PgUp</kbd> and <kbd>PgDn</kbd> keys
    or triggering equivalent operations from the scrollbar)
    so that within the <a>optimal viewing region</a> of the <a>scrollport</a>
    the user sees a continuous stream of content.

    For a <a>scroll snap container</a> this region also defines
    the <dfn local-lt="snapport">scroll snapport</dfn>--
    the area of the scrollport that is used as the <a>alignment container</a>
    for the <a>scroll snap areas</a> when calculating <a>snap positions</a>.

    <div class="example">
        In this example, 'scroll-padding' is used to center slideshow images
        within the portion of the scrollport
        that is not obscured by a fixed-position toolbar.

        <pre class="lang-css">
            body {
                overflow-x: auto;
                overflow-y: hidden;
                scroll-snap-type: x mandatory;
                scroll-padding: 0 500px 0 0;
            }
            .toolbar {
                position: fixed;
                height: 100%;
                width: 500px;
                right: 0;
            }
            img {
                scroll-snap-align: center none;
            }
        </pre>
    </div>

Aligning Scroll Snap Areas:  Properties on the elements {#properties-on-the-elements}
=====================================================================================

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Scroll Snapping Area: the 'scroll-margin' property {#scroll-margin}
-------------------------------------------------------------------

    <pre class="propdef">
    Name: scroll-margin
    Value: <<length>>{1,4}
    Initial: 0
    Applies to: all elements
    Inherited: no
    Percentages: n/a
    Media: interactive
    Computed value: as specified, with lengths made absolute
    Animatable: as length
    </pre>

    This property is a <a>shorthand property</a> that sets
    all of the <a href="#longhands"><css>scroll-margin-*</css> longhands</a>
    in one declaration,
    assigning values to the longhands representing each side
    exactly as the 'margin' property does for its longhands.

    Values represent outsets defining the
    <dfn lt="scroll snap area" local-lt="snap area">scroll snap area</dfn>
    that is used for snapping this box to the snapport.
    The <a>scroll snap area</a> is the rectangular bounding box of the transformed border box,
    plus the specified outsets,
    axis-aligned in the <a>scroll container’s</a> coordinate space.

    Note: This ensures that the <a>scroll snap area</a> is always rectangular
    and axis-aligned to the <a>scroll container’s</a> coordinate space.

    If a page is navigated to a fragment that defines a target element
    (one that would be matched by '':target'',
    or the target of {{scrollIntoView()}}),
    the UA should use the element's <a>scroll snap area</a>,
    rather than just its border box,
    to determine which area of the <a>scrollable overflow region</a>
    to bring into view.

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Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-align}
--------------------------------------------------------------------------------

    <pre class="propdef">
    Name: scroll-snap-align
    Value: [ none | start | end | center ]{1,2}
    Initial: none
    Applies to: all elements
    Inherited: no
    Percentages: n/a
    Media: interactive
    Computed value: two keywords
    Animatable: no
    </pre>

    The 'scroll-snap-align' property specifies
    the box’s <a>snap position</a> as an alignment of
    its <a>snap area</a> (as the <a>alignment subject</a>)
    within its <a>snap container’s</a> <a>snapport</a> (as the <a>alignment container</a>).
    The two values specify the snapping alignment
    in the <a>inline axis</a> and <a>block axis</a>, respectively.
    If only one value is specified, the second value defaults to the same value.

    Values are defined as follows:

    <dl dfn-type="value" dfn-for="scroll-snap-align">
        <dt><dfn>none</dfn>
        <dd>
            This box does not define a <a>snap position</a> in the specified axis.

        <dt><dfn>start</dfn>
        <dd>
            Start alignment of this box’s <a>scroll snap area</a>
            within the <a>scroll container</a>’s <a>snapport</a>
            is a <a>snap position</a>
            in the specified axis.

        <dt><dfn>end</dfn>
        <dd>
            End alignment of this box’s <a>scroll snap area</a>
            within the <a>scroll container</a>’s <a>snapport</a>
            is a <a>snap position</a>
            in the specified axis.

        <dt><dfn>center</dfn>
        <dd>
            Center alignment of this box’s <a>scroll snap area</a>
            within the <a>scroll container</a>’s <a>snapport</a>
            is a <a>snap position</a>
            in the specified axis.
    </dl>

<h4 id="snap-scope">
Scoping Valid Snap Positions to Visible Boxes</h4>

    Since the purpose of scroll snapping is to align content within the viewport
    for optimal viewing,
    a <a>scroll container</a> cannot be <a>snapped</a> to a <a>snap area</a>
    if no part of it is within the <a>snapport</a>.
    For example, a <a>snap area</a> is top-aligned to the <a>snapport</a>
    if its top edge is coincident with the <a>snapport</a>’s top edge;
    however, this scroll position is nonetheless not a valid <a>snap position</a>
    if the entire <a>snap area</a> is outside the <a>snapport</a>
    (because the required alignment, though satisfied, would not be relevant to the viewer).

    <figure>
        <pre class="ascii-art">
            ╔════viewport════╗┈┈┈┈┈┈┈┈┌──────────────┐
            ║  ┌─────┐ ┌──┐  ║        │ top-snapping │
            ║  ├──┐  │ └──┘  ║        │   element    │
            ║  └──┴──┘       ║        │              │
            ╚════════════════╝        │              │
                                      └──────────────┘
        </pre>
        <figcaption>
          Alignment of an off-screen element
          is not considered <a>snapping</a>.
        </figcaption>
    </figure>

    <details class="note">
        <summary>Why limit snapping to only when the element is visible?</summary>
        As the <a href="https://www.webkit.org/blog/4017/scroll-snapping-with-css-snap-points/">WebKit implementers point out</a>,
        extending a snap edge infinitely across the canvas
        only allows for snapping gridded layouts,
        and produces odd behavior for the user
        when off-screen elements do not align
        with on-screen elements.
        (If this requirement is onerous for implementers however,
        we can default to a gridded behavior
        and introduce a switch to get smarter behavior later.)
    </details>

    Note: Although ''scroll-snap-type: both'' evaluates [=snap positions=] independently in each axis,
    <a href="#choosing">choosing</a> of a [=snap position=] in one axis
    can be influenced by [=snap positions=] in the other axis.
    For example, snapping in one axis
    may push off-screen the [=snap area=] that the other axis would otherwise align to,
    making its [=snap position=] invalid and therefore unchooseable.

<h4 id="snap-overflow">
Snapping Boxes that Overflow the Scrollport</h4>

    If the <a>snap area</a> is larger than the <a>snapport</a> in a particular axis,
    then any scroll position in which the <a>snap area</a> covers the <a>snapport</a>,
    and the distance between the geometrically previous and subsequent <a>snap positions</a> in that axis
    is larger than size of the <a>snapport</a> in that axis,
    is a valid <a>snap position</a> in that axis.
    The UA may use the specified alignment as a more precise target
    for certain scroll operations (e.g. explicit paging).

    <div class="example">
        For example, take the first example in [[#examples]],
        which had a photo as the area.
        The author wants mandatory snapping from item to item,
        but if the item happens to be larger than your viewport,
        you want to be able to scroll around the whole thing once you’re over it.

        Since the <a>snap area</a> is larger than the <a>snapport</a>,
        while the area fully fills the viewport,
        the container can be scrolled arbitrarily
        and will not try to snap back to its aligned position.
        However, if the container is scrolled such that the area
        no longer fully fills the viewport in an axis,
        the area resists outward scrolling
        until it is scrolled sufficiently
        to trigger snapping to a different <a>snap position</a>.
    </div>

    <div class=example>
        For another example,
        mandatory top-snapping on nested <{section}> elements
        can produce large snapping areas
        (from large top-level sections)
        potentially filled with smaller snapping areas
        (from the subsections).
        When the subsections are small enough,
        they snap normally;
        when they're longer,
        the viewer can scroll arbitrarily within them,
        or within a large segment of the top-level section that has no subsections to snap to.

        <figure>
            <pre class=ascii-art>
            ┌─ top-level section ─┐ ━┓
            │                     │ 1┃
            │                     │  ┃
            │                     │ ━┩
            │                     │  ┆
            │                     │  ┆
            │┌─── sub-section ───┐│  ╯ ━┓
            │└───────────────────┘│    2┃
            │┌─── sub-section ───┐│ ━┓  ┃
            ││                   ││ 3┃ ━┛
            │└───────────────────┘│  ┃
            │┌─── sub-section ───┐│ ━┛ ━┓
            │└───────────────────┘│    4┃
            │┌─── sub-section ───┐│ ━┓  ┃
            ││                   ││ 5┃ ━┛
            ││                   ││  ┃
            ││                   ││ ━┩
            ││                   ││  ┆
            ││                   ││  ┆
            ││                   ││  ┆
            │└───────────────────┘│  ┆
            └─────────────────────┘  ╯
            </pre>
            <figcaption>
                In the figure above,
                the five numbered viewports
                represent the five snap positions
                associated with the top-level section
                and its four subsections.
                Because the first and last snap positions are part of ranges taller than the viewport,
                the viewer is allowed to scroll freely
                between the top and bottom of each range.
            </figcaption>
        </figure>

        Note: If the author had instead set mandatory snap positions
        on the <em>headings</em> of each section
        (rather than the sections themselves),
        the contents of the first and fifth sections
        would be partially inaccessible to the user,
        as the heading snap area does not extend to cover the whole section.
        This is why it's a bad idea to use mandatory snap positions
        on elements that might be widely spaced apart.
    </div>


<h4 id="unreachable">
Unreachable Snap Positions</h4>

    If a <a>snap position</a> is unreachable as specified,
    such that aligning to it would require scrolling the <a>scroll container</a>’s viewport
    past the edge of its <a>scrollable overflow region</a>,
    the <em>used</em> <a>snap position</a> for this <a>snap area</a>
    is the position resulting from scrolling <em>as much as possible</em>
    in each relevant axis
    toward the desired <a>snap position</a>.

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Scroll Snap Limits: the 'scroll-snap-stop' property {#scroll-snap-stop}
--------------------------

    <pre class="propdef">
    Name: scroll-snap-stop
    Value: normal | always
    Initial: normal
    Applies to: all elements
    Inherited: no
    Percentages: n/a
    Computed value: as specified
    Animatable: no
    Media: interactive
    </pre>

    When scrolling with an intended direction,
    the <a>scroll container</a> can “pass over” several possible <a>snap positions</a>
    (that would be valid to snap to,
    if the scrolling operation used the same direction
    but a lesser distance)
    before reaching the natural endpoint of the scroll operation
    and selecting its final <a>scroll position</a>.
    The 'scroll-snap-stop' property allows such a possible <a>snap position</a>
    to “trap” the scrolling operation,
    forcing the <a>scroll container</a> to stop
    before the scrolling operation would naturally end.

    Values are defined as follows:

    <dl dfn-type=value dfn-for=scroll-snap-stop>
        <dt><dfn>normal</dfn>
        <dd>
            The <a>scroll container</a> may pass over a <a>snap position</a> defined
            by this element during the execution of a scrolling operation.

        <dt><dfn>always</dfn>
        <dd>
            The <a>scroll container</a> must not pass over a <a>snap position</a>
            defined by this element during the execution of a scrolling operation;
            it must instead snap to the first of this element's <a>snap positions</a>.
    </dl>

    This property has no effect on scrolling operations with only an <a>intended end position</a>,
    as they do not conceptually “pass over” any <a>snap positions</a>.

Snapping Mechanics {#snap-concepts}
===================================

    The precise model algorithm to select a <a>snap position</a> to snap to
    is intentionally left mostly undefined,
    so that user agents can take into account sophisticated models of user intention and interaction
    and adjust how they respond over time,
    to best serve the user.

    This section defines some useful concepts to aid in discussing scroll-snapping mechanics,
    and provides some guidelines for what an effective scroll-snapping strategy might look like.
    User agents are encouraged to adapt this guidance
    and apply their own best judgement
    when defining their own snapping behavior.
    It also provides a small number of behavior requirements,
    to ensure a minimum reasonable behavior that authors can depend on
    when designing their interfaces with scroll-snapping in mind.


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Types of Scrolling Methods {#scroll-types}
------------------------------------------

    When a page is scrolled,
    the action is performed with
    an intended end position
    and/or an intended direction.
    Each combination of these two things
    defines a distinct category of scrolling,
    which can be treated slightly differently:

    : <dfn export>intended end position</dfn>
    ::
        <div class="example">
            Common examples of scrolls with only an <a>intended end position</a> include:

            * a panning gesture,
                released without momentum
            * manipulating the scrollbar “thumb” explicitly
            * programmatically scrolling via APIs such as {{Window/scrollTo()}}
            * tabbing through the document’s focusable elements
            * navigating to an anchor within the page
            * homing operations such as the <kbd>Home</kbd>/<kbd>End</kbd> keys
        </div>

    : <dfn export>intended direction and end position</dfn>
    ::
        <div class="example">
            Common examples of scrolls with both an <a>intended direction and end position</a> include:

            * a “fling” gesture,
                interpreted with momentum
            * programmatically scrolling via APIs such as {{Window/scrollBy()}}
            * paging operations such as the <kbd>PgUp</kbd>/<kbd>PgDn</kbd> keys (or equivalent operations on the scrollbar)
        </div>

        The intended end point of the scroll prior to intervention from features such
        as snap points is its <dfn noexport>natural end-point</dfn>.

    : <dfn export>intended direction</dfn>
    ::
        <div class="example">
            Common examples of scrolls with only an <a>intended direction</a> include:

            * pressing an arrow key on the keyboard
            * a swiping gesture interpreted as a fixed (rather than inertial) scroll
        </div>

    Additionally, because page layouts usually align things vertically and/or horizontally,
    UAs sometimes <dfn export>axis-lock</dfn> a scroll when its direction
    is sufficiently vertical or horizontal.
    An <a>axis-locked</a> scroll is bound to only scroll along that axis.
    This prevents less-precise input mechanisms from drifting in the non-primary axis.

    Note: This specification only applies to scrolling methods supported by the user agent;
    it does not require the user agent to support any particular input or scrolling method.

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Choosing Snap Positions {#choosing}
-----------------------------------

    A <a>scroll container</a> can have many <a>snap areas</a>
    scattered throughout its <a>scrollable overflow region</a>.
    A naïve algorithm for selecting a <a>snap position</a>
    can produce behavior that is unintuitive for users,
    so care is required when designing a selection algorithm.
    Here are a few pointers that can aid in the selection process:

    * <a>Snap positions</a> should be chosen to minimize the distance between the end-point
        (or the <a>natural end-point</a>)
        and the final snapped scroll position,
        subject to the additional constraints listed in this section.

    * If a scroll is <a>axis-locked</a>,
        any <a>snap positions</a> in the other axis should be ignored
        during the scroll.
        (However, <a>snap positions</a> in the other axis can still effect the final scroll position.)

    * In order to prevent a far-offscreen element
        from having difficult-to-understand effects
        on the scroll position,
        <a>snap positions</a> should be ignored if their elements are far outside of the “corridor”
        that the <a>snapport</a> defines as it moves through the <a>scrollable overflow region</a>,
        or a hypothetical “corridor” in the direction of a scroll with only an <a>intended direction</a>,
        or the <a>snapport</a> after an scroll with only an <a>intended end position</a>.

    * User agents <em>must</em> ensure that a user can “escape” a <a>snap position</a>,
        regardless of the scroll method.
        For example, if the snap type is ''mandatory''
        and the next <a>snap position</a> is more than two screen-widths away,
        a naïve “always snap to nearest” selection algorithm might “trap” the user
        if their end position was only one screen-width away.
        Instead, a smarter algorithm that only returned to the starting <a>snap position</a>
        if the end-point was a fairly small distance from it,
        and otherwise ignored the starting snap position,
        would give better behavior.

        (This implies that a scroll with only an <a>intended direction</a>
        must always ignore the starting <a>snap positions</a>.)

    * If a page is navigated to a fragment that defines a target element
        (one that would be matched by '':target'',
        or the target of {{Element/scrollIntoView()}}),
        and that element defines some <a>snap positions</a>,
        the user agent must <a>snap</a> to one of that element’s <a>snap positions</a>
        if its nearest <a>scroll container</a> is a <a>scroll snap container</a>.
        The user agent <em>may</em> also do this even when the <a>scroll container</a> has ''scroll-snap-type: none''.

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Appendix A: Longhands {#longhands}
==================================

The physical and logical longhands (and their shorthands)
interact as defined in [[!CSS-LOGICAL-1]].

Physical Longhands for 'scroll-padding' {#padding-longhands-physical}
----------------------------------------------------------------------

    <pre class="propdef">
    Name: scroll-padding-top, scroll-padding-right, scroll-padding-bottom, scroll-padding-left
    Value: <<length-percentage>>
    Initial: 0
    Applies to: <a>scroll containers</a>
    Inherited: no
    Percentages: relative to the scroll container’s scrollport
    Media: interactive
    Computed value: as specified, with lengths made absolute
    Animatable: as length, percentage, or calc
    </pre>

    These <a>longhands</a> of 'scroll-padding' specify
    the top, right, bottom, and left edges
    of the <a>snapport</a>, respectively.

Flow-relative Longhands for 'scroll-padding'  {#padding-longhands-logical}
--------------------------------------------------------------------------

    <pre class="propdef">
    Name: scroll-padding-inline-start, scroll-padding-block-start, scroll-padding-inline-end, scroll-padding-block-end
    Value: <<length-percentage>>
    Initial: 0
    Applies to: <a>scroll containers</a>
    Inherited: no
    Percentages: relative to the scroll container’s scrollport
    Media: interactive
    Computed value: as specified, with lengths made absolute
    Animatable: as length, percentage, or calc
    </pre>

    These <a>longhands</a> of 'scroll-padding' specify
    the block-start, inline-start, block-end, and inline-end edges
    of the <a>snapport</a>, respectively.

    <pre class="propdef">
    Name: scroll-padding-block, scroll-padding-inline
    Value: <<length-percentage>>{1,2}
    Initial: 0
    Applies to: all elements
    Inherited: no
    Percentages: relative to the scroll container’s scrollport
    Media: interactive
    Computed value: as specified, with lengths made absolute
    Animatable: as length, percentage, or calc
    </pre>

    These <a>shorthands</a> of 'scroll-padding-block-start' + 'scroll-padding-block-end'
    and 'scroll-padding-inline-start' + 'scroll-padding-inline-end'
    are <a>longhands</a> of 'scroll-padding',
    and specify the block-axis and inline-axis edges of the <a>snapport</a>, respectively.

    If two values are specified, the first gives the start value
    and the second gives the end value.

Physical Longhands for 'scroll-margin'  {#margin-longhands-physical}
--------------------------------------------------------------------

    <pre class="propdef">
    Name: scroll-margin-top, scroll-margin-right, scroll-margin-bottom, scroll-margin-left
    Value: <<length>>
    Initial: 0
    Applies to: all elements
    Inherited: no
    Percentages: n/a
    Media: interactive
    Computed value: as specified, with lengths made absolute
    Animatable: as length
    </pre>

    These <a>longhands</a> of 'scroll-margin' specify
    the top, right, bottom, and left edges
    of the <a>scroll snap area</a>, respectively.

Flow-relative Longhands for 'scroll-margin'  {#margin-longhands-logical}
------------------------------------------------------------------------

    <pre class="propdef">
    Name: scroll-margin-block-start, scroll-margin-inline-start, scroll-margin-block-end, scroll-margin-inline-end
    Value: <<length>>
    Initial: 0
    Applies to: all elements
    Inherited: no
    Percentages: n/a
    Media: interactive
    Computed value: as specified, with lengths made absolute
    Animatable: as length
    </pre>

    These <a>longhands</a> of 'scroll-margin' specify the
    block-start, inline-start, block-end, and inline-end edges
    of the <a>scroll snap area</a>, respectively.

    <pre class="propdef">
    Name: scroll-margin-block, scroll-margin-inline
    Value: <<length>>{1,2}
    Initial: 0
    Applies to: all elements
    Inherited: no
    Percentages: n/a
    Media: interactive
    Computed value: as specified, with lengths made absolute
    Animatable: as length
    </pre>

    These <a>shorthands</a> of 'scroll-margin-block-start' + 'scroll-margin-block-end'
    and 'scroll-margin-inline-start' + 'scroll-margin-inline-end'
    are <a>longhands</a> of 'scroll-margin',
    and specify the block-axis and inline-axis edges
    of the <a>scroll snap area</a>, respectively.

    If two values are specified, the first gives the start value
    and the second gives the end value.

Privacy and Security Considerations {#priv-sec}
===============================================

    This specification does not expose any information whatsoever
    that is not already exposed to the DOM directly;
    it just makes scrolling slightly more functional.
    There are no new privacy or security considerations.

Acknowledgements {#acknowledgements}
====================================

    Many thanks to
    David Baron,
    Simon Fraser,
    Håkon Wium Lie,
    Edward O’Connor,
    François Remy,
    Majid Valpour,
    and most especially Robert O’Callahan
    for their proposals and recommendations,
    which have been incorporated into this document.

Changes {#changes}
==================

Changes Since 24 August 2017 CR {#changes-20170824}
---------------------------------------------------

	Changes since the <a href="https://www.w3.org/TR/2017/CR-css-scroll-snap-1-20170824/">24 August 2017 Candidate Recommendation</a> include:

	<ul>
		<li id="change-2017-scroll-margin">
			'':target''/{{scrollIntoView()}}/etc should take 'scroll-margin' into account,
			regardless of whether snapping is turned on or not.
			(<a href="https://drafts.csswg.org/css-scroll-snap-1/issues-cr-2016-08#issue-1">Issue 1</a>
			<blockquote>
				<p><ins>If a page is navigated to a fragment that defines a target element
				(one that would be matched by '':target'',
				or the target of {{scrollIntoView()}}),
				the UA should use the element's <a>scroll snap area</a>,
				rather than just its border box,
				to determine which area of the <a>scrollable overflow region</a>
				to bring into view.</ins></p>
			</blockquote>

		<li id="change-2017-target-snap-must">
			'':target''/{{scrollIntoView()}}/etc must (rather than should)
			use snap positions if snapping is turned on.
			(<a href="https://drafts.csswg.org/css-scroll-snap-1/issues-cr-2016-08#issue-1">Issue 1</a>
			<blockquote>
				<p>If a page is navigated to a fragment that defines a target element
				(one that would be matched by '':target'',
				or the target of {{Element/scrollIntoView()}}),
				and that element defines some <a>snap positions</a>,
				the user agent <del>should</del> <ins>must</ins> <a>snap</a>
				to one of that element’s <a>snap positions</a>
				<ins>if its nearest <a>scroll container</a> is a <a>scroll snap container</a></ins>.
				The user agent <em>may</em> also do this even when
				the <a>scroll container</a> has ''scroll-snap-type: none''.</p>
			</blockquote>

		<li id="change-2017-rename-scroll-margin">
			Renamed <css>scroll-snap-margin</css> to 'scroll-margin'
			to reflect its more generic role in providing breathing space
			for scrolling to an element regardless of snapping behavior.
			(<a href="https://drafts.csswg.org/css-scroll-snap-1/issues-cr-2016-08#issue-4">Issue 4</a>)

	A <a href="https://drafts.csswg.org/css-scroll-snap-1/issues-cr-2016-08">Disposition of Comments</a> is available.

Changes Since 20 October 2016 CR {#changes-20161020}
----------------------------------------------------

	Changes since the <a href="https://www.w3.org/TR/2016/CR-css-scroll-snap-1-20161020/">20 October 2016 Candidate Recommendation</a> include:

	<ul>
		<li>Restricted 'scroll-padding' to non-negative values.
			(<a href="https://github.com/w3c/csswg-drafts/issues/1084">Issue 1084</a>)
			<blockquote>
				<p>Values <ins>must be non-negative and</ins>
				are interpreted as for 'padding' &hellip;</p>
			</blockquote>
		<li>Added paging and homing operations to examples in <a href="#scroll-types"></a>.
			(<a href="https://github.com/w3c/csswg-drafts/issues/1605">Issue 1605</a>)
		<li>Clarified that snapping in one axis may affect whether snapping to a particular snap area is possible in the other axis.
			(<a href="https://github.com/w3c/csswg-drafts/issues/950">Issue 950</a>)
			<blockquote>
                <ins><p class="note">Although ''scroll-snap-type: both'' evaluates [=snap positions=] independently in each axis,
                <a href="#choosing">choosing</a> of a [=snap position=] in one axis
                may be influenced by [=snap positions=] in the other axis.
                For example, snapping in one axis
                may push off-screen the [=snap area=] that the other axis would otherwise align to,
                making its [=snap position=] invalid and therefore unchooseable.</p></ins>
			</blockquote>
		<li>Clarified how the 'scroll-padding' and 'scroll-margin' shorthands
			assign values to their longhands.
			(<a href="https://github.com/w3c/csswg-drafts/issues/1050">Issue 1050</a>)
		<li>Clarified that scroll snapping does not mandate any particular input method.
			(<a href="https://github.com/w3c/csswg-drafts/issues/1305">Issue 1305</a>)
            <blockquote>
				<ins><p class="note">This specification only applies to scrolling methods supported by the user agent;
				it does not require the user agent to support any particular input or scrolling method.</p></ins>
			</blockquote>
		<li>Clarified the intended effects of 'scroll-snap-stop' on various scrolling operations.
			(<a href="https://github.com/w3c/csswg-drafts/issues/1552">Issue 1552</a>)
		<li>Clarified that 'scroll-snap-stop' is applied to the <a>snap positions</a> defined by the element,
			not applied to all <a>snap positions</a> in the <a>scroll snap container</a>.
		<li>Fixed some syntax errors in examples and added a new one to the 'scroll-snap-type' section.
			(<a href="https://github.com/w3c/csswg-drafts/issues/827">Issue 827</a>)
	</ul>


	A <a href="https://drafts.csswg.org/css-scroll-snap-1/issues-cr-2016">Disposition of Comments</a> is available.