This module contains features to control panning and scrolling behavior with “snap positions”.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
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This is a public copy of the editors’ draft.
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This section is not normative.
This is currently a draft spec over Scroll Snap 1.
Scroll experiences don’t always start at the beginning. Interactions with
carousels, swipe controls, and listviews often start somewhere in the middle,
By enabling CSS to specify this scroll start position,
both users, page authors and browsers benefit.
In addition to setting an initial scroll position,
developers need insights and events into Scroll Snap.
Events like which element is snapped on which axis,
when the snap event is changing,
when snap completes and conveniences for
snapping to children programmatically.
1.1. First Layout
This event should follow the Animation code path. When animation objects are created and fire events, this is when a box has it’s first layout.
This property is a shorthand property that sets all of the scroll-start-* longhands in one declaration.
The first value defines the scroll starting point in the block axis,
the second sets it in the inline axis. If the second value is omitted, it defaults to start. If scroll-start-target is set on any child, scroll-start is not used, in favor of using the element as the offset.
3.1.2. Slow page loading or document streaming behavior
3.1.3. Interaction with fragment navigation
If the scrollport has a in-page :target via a URL fragment or a previous scroll position, then scroll-start is unused. Existing target logic should go unchanged. This makes scroll-start a soft request in the scroll position resolution routines.
3.1.4. Interaction with place-content
Side note: While place-content can make a scroller appear to start in the center
or end, no browser supports it and it appears complicated to resolve.
3.1.5. Interaction with "find in page"
3.1.6. Interaction scroll-snap containers
This effectively will layout and start scroll at the snapped child, thus negating / cancelling scroll-start. scroll-start will only work if nothing else has effected the scroll position.
This property is a shorthand property that sets all of the scroll-start-target-* longhands in one declaration.
The first value defines the scroll starting point in the block axis,
the second sets it in the inline axis.
If the second value is omitted, it defaults to none.
Values are defined as follows:
Element is not a scroll-start-target.
Element is used to calculate the scroll-start position,
taking into account scroll-padding or scroll-margin ,
same as a scroll-snap target.
The :snapped pseudo-class matches any scroll snap targets,
regardless of axis.
The longform physical and logical pseudo-class selectors
allow for more finite snapped children styling
as they can target an individual axis.
CSS scroll snap points are often used as a mechanism to
create scroll interactive "selection" components,
and a scroll end guestimate. By creating a built-in event,
the invisible state will become actionable,
at the right time, and always correct.
Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with class="example",
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the
normative text with class="note", like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class="advisement">, like
this: UAs MUST provide an accessible alternative.
Tests relating to the content of this specification
may be documented in “Tests” blocks like this one.
Any such block is non-normative.
Conformance to this specification
is defined for three conformance classes:
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
Implementations of Unstable and Proprietary Features
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS