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CSS Exclusions define arbitrary areas around which inline content ([CSS21]) can flow. CSS Exclusions can be defined on any CSS block-level elements. CSS Exclusions extend the notion of content wrapping previously limited to floats.
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This specification was previously called “CSS Exclusions and Shapes.” The definitions for shapes are now in a separate specification, see [CSS-SHAPES].
This section is not normative.
This specification defines features that allow inline flow content to wrap around outside the exclusion area of elements.
Exclusion box
A box ([CSS3BOX]) that defines an exclusion area for other boxes. The ‘wrap-flow
’ property is
used to make an element's generated box an exclusion box. An exclusion box
contributes its exclusion area to its
containing block's wrapping context.
An element with a ‘float
’ computed value
other than ‘none
’ does not become an
exclusion.
Exclusion area
The area used for excluding inline flow content around an exclusion box. The exclusion area is equivalent to the border box for an exclusion box.
If a user agent implements both CSS Exclusions and CSS Shapes, the shape-outside property defines the exclusion area instead of the border box.
Exclusion element
An block-level element which is not a float and generates an exclusion box. An element generates an exclusion
box when its ‘wrap-flow
’ property's computed value is not
‘auto
’.
Wrapping context
should the wrapping context be generic and include floats?
The wrapping context of a box is a
collection of exclusion areas contributed by
its associated exclusion boxes. During
layout, a box wraps its inline flow content in the wrapping area
that corresponds to
the subtraction of its wrapping context
from its own content area.
A box inherits its containing
block's wrapping context unless it
specifically resets it using the ‘wrap-through
’ property.
Content area
The content area is normally used for layout of the inline flow content of a box.
Wrapping area
The area used for layout of inline flow content of a box affected by a wrapping context, defined by subtracting the wrapping context from its content area
Outside and inside
In this specification, ‘outside
’ refers
to DOM content that is not a descendant of an element while ‘inside
’ refers to the element's descendants.
Exclusion elements define exclusion areas that contribute to their containing block's wrapping context. As a consequence, exclusions impact the layout of their containing block's descendants.
Elements layout their inline content in their content area and wrap around the exclusion areas in their associated wrapping context. If the element is itself an
exclusion, it does not wrap around its own exclusion area and the impact
of other exclusions on other exclusions is controlled by the ‘z-index
’ property as explained in the exclusions order section.
An element becomes an exclusion when its ‘wrap-flow
’ property has a computed value other
than ‘auto
’.
wrap-flow
’ propertyName: | wrap-flow |
---|---|
Value: | auto | both | start | end | minimum | maximum | clear |
Initial: | auto |
Applies to: | block-level elements. |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | as specified except for element's whose ‘float ’ computed value is not none , in which case the computed value is ‘auto ’.
|
The values of this property have the following meanings:
If the property's computed value is ‘auto
’, the element does not become an
exclusion.
Otherwise, a computed ‘wrap-flow
’ property value of ‘both
’, ‘start
’,
‘end
’, ‘minimum
’, ‘maximum
’ or ‘clear
’ on an element makes that element an exclusion element. It's exclusion area is contributed to its containing
block's wrapping context, causing the
containing block's descendants to wrap around its exclusion area.
Exclusion with ‘wrap-flow:
start
’ interacting with various writing modes.
Determining the relevant edges of the exclusion depends on the writing mode [CSS3-WRITING-MODES] of the content wrapping around the exclusion area.
An exclusion element establishes a new block formatting context (see [CSS21]) for its content.
Combining exclusions
The above figure illustrates how exclusions are combined. The outermost
box represents an element's content box. The A, B, C and D darker gray
boxes represent exclusions in the element's wrapping context. A, B, C and D have their
respective ‘wrap-flow
’ computed to ‘both
’, ‘start
’,
‘end
’ and ‘clear
’ respectively. The lighter gray areas show
the additional areas that are excluded for inline layout as a result of
the ‘wrap-flow
’value. For example, the area to the
right of ‘B
’ cannot be used for inline
layout of left-to-right writing mode content because the ‘wrap-flow
’ for
‘B
’ is ‘start
’.
The background ‘blue
’ area shows what
areas are available for a left-to-right writing mode element's inline
content layout. All areas represented with a light or dark shade of gray
are not available for (left-to-right writing mode) inline content layout.
The ‘wrap-flow
’ property values applied to
exclusions as grid items.
<div id="grid"> <div id="top-right" class="exclusion"></div> <div id="bottom-left" class="exclusion"></div> <div id="content">Lorem ipsum…</div> </div> <style type="text/css"> #grid { width: 30em; height: 30em; display: grid; grid-columns: 25% 25% 25% 25%; grid-rows: 25% 25% 25% 25%; #top-right { grid-column: 3; grid-row: 2; } #bottom-left { grid-column: 2; grid-row: 3; } .exclusion { wrap-flow: <see below> } #content { grid-row: 1; grid-row-span: 4; grid-column: 1; grid-column-span: 4; } </style>
The following figures illustrate the visual rendering for different
values of the ‘wrap-flow
’ property. The gray grid lines are
marking the grid cells. and the blue area is the exclusion box
(positioned by the grid).
.exclusion{ wrap-flow: auto; }
| .exclusion{ wrap-flow: both; }
|
.exclusion{ wrap-flow: start; }
| .exclusion{ wrap-flow: end; }
|
.exclusion{ wrap-flow: minimum; }
| .exclusion{ wrap-flow: maximum; }
|
.exclusion{ wrap-flow: clear; }
| |
An exclusion affects the inline flow content descended
from the exclusion's containing block (defined in
CSS 2.1 10.1) and that of all descendant elements of the same
containing block. All inline flow content inside the containing block of
the exclusions is affected. To stop the effect of exclusions defined
outside an element, the ‘wrap-through
’ property can be used (see the propagation of exclusions section
below).
As a reminder, for exclusions with ‘position:fixed
’, the containing block is that of the
root element.
By default, an element inherits its parent wrapping context. In other words it is subject to the exclusions defined outside the element.
Setting the ‘wrap-through
’ property to ‘none
’ prevents an element from inheriting its
parent wrapping context. In other words,
exclusions defined ‘outside
’ the element,
have not effect on the element's children layout.
wrap-through
’ computes to none, or the element
itself, then exclusion still have an effect on the children of that
containing block element.wrap-through
’
PropertyName: | wrap-through |
---|---|
Value: | wrap | none |
Initial: | wrap |
Applies to: | block-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | as specified |
The values of this property have the following meanings:
Using the ‘wrap-through
’ property to control the effect
of exclusions.
<style type="text/css">
#grid {
display: grid;
grid-columns: 25% 50% 25%;
grid-rows: 25% 25% 25% 25%;
}
#exclusion {
grid-row: 2;
grid-row-span: 2;
grid-column: 2;
wrap-flow: <see below>
}
#rowA, #rowB {
grid-row-span: 2;
grid-column: 1;
grid-column-span: 3;
}
#rowA {
grid-row: 1;
}
#rowB {
grid-row: 3;
}
</style>
<style type="text/css">
.exclusion {
wrap-flow: both;
position: absolute;
left: 20%;
top: 20%;
width: 50%;
height: 50%;
background-color: rgba(220, 230, 242, 0.5);
}
</style>
<div id="grid">
<div class=”exclusion”></div>
<div id="rowA" style=”wrap-through: wrap;”> Lorem ipsum dolor sit amet...</div>
<div id="rowB" style=”wrap-through: none;”> Lorem ipsum dolor sit amet...</div>
</div>
Exclusions follow the painting order (See [CSS21] Appendix E). Exclusions are
applied in reverse to the document order in which they are defined. The
last exclusion appears on top of all other exclusion, thus it affects the
inline flow content of all other preceding exclusions or elements
descendant of the same containing block. The ‘z-index
’ property can be used to change the
ordering of positioned
exclusion boxes (see [CSS21]). Statically positioned
exclusions are not affected by the ‘z-index
’ property and thus follow the painting
order.
Ordering of exclusions.
<style type="text/css">
.exclusion {
wrap-flow: both;
position: absolute;
width: 200px;
}
.topleft {
top: 10px;
left: 10px;
background-color: lightblue;
}
.middle {
top: 90px;
left: 90px;
background-color: lightgreen;
}
.bottomright {
top: 170px;
left: 170px;
background-color: pink;
}
</style>
<div class="exclusion topleft">
The top left div...
</div>
<div class="exclusion middle">
The middle div...
</div>
<div class="exclusion bottomright">
The bottom right div...
</div>
.middle { z-index: auto;
}
| .middle { z-index: 1; }
|
Is the CSS exclusions processing model incorrect?
The current draft provides a model for exclusions without a collision-avoidance model. The existing exclusion model in CSS uses floats, which have both exclusion and collision-avoidance behavior. Concerns have been raised that allowing exclusions without collision avoidance could be harmful, particularly with absolutely-positioned elements. Three options should be considered:
Applying exclusions is a two-step process:
In this step, the user agent determines which containing
block each exclusion area belongs to.
This is a simple step, based on the definition of containing blocks and
elements with a computed value for ‘wrap-flow
’ that is not auto
.
In this step, starting from the top of the rendering tree (see [CSS21]), the the agent processes each containing block in two sub-steps.
Resolving the position and size of exclusion boxes in the wrapping context may or may not require a layout. For example, if an exclusion box is absolutely positioned and sized, a layout may not be needed to resolve its position and size. In other situations, laying out the containing block's content is required.
When a layout is required, it is carried out without applying any exclusion area. In other words, the containing block is laid out without a wrapping context.
Step 2-A yields a position and size for all exclusion boxes in the wrapping context. Each exclusion box is processed in turn, starting from the top-most, and each exclusion area is computed and contributed to the containing block's wrapping context.
Scrolling is ignored in this step when resolving the position and size
of ‘position:fixed
’ exclusion boxes.
Once the containing block's wrapping context is computed, all exclusion boxes in that wrapping context are removed from the normal flow.
Finally, the content of the containing block is laid out, with the
inline content wrapping around the wrapping content‘s exclusion areas
When the containing block itself is an exclusion box, then rules on exclusions order define which exclusions affect the inline and descendant content of the box.
This section illustrates the exclusions processing model with an example. It is meant to be simple. Yet, it contains enough complexity to address the issues of layout dependencies and re-layout.
The code snippet in the following example has two exclusions affecting the document’s inline content.
<html> <style> #d1 { position:relative; height: auto; color: #46A4E9; border: 1px solid gray; } #e1 { wrap-flow: both; position: absolute; left: 50%; top: 50%; width: 40%; height: 40%; border: 1px solid red; margin-left: -20%; margin-top: -20%; } #d2 { position: static; width: 100%; height: auto; color: #808080; } #e2 { wrap-flow: both; position: absolute; right: 5ex; top: 1em; width: 12ex; height: 10em; border: 1px solid lime; } </style> <body> <div id="d1"> Lorem ipsusm ... <p id="e1"></p> </div> <div id="d2"> Lorem ipsusm ... <p id="e2" ></p> </div> </body> </html>
The following figures illustrate:
DOM tree
Layout tree of generated block boxes
The figures illustrate how the boxes corresponding to the element
sometimes have a different containment hierarchy in the layout tree than
in the DOM tree. For example, the box generated by e1
is positioned in its containing block's box, which is
the d1-box
, because e1
is
absolutely positioned and d1
is relatively
positioned. However, while e2
is also absolutely
positioned, its containing block is the initial containing block (ICB).
See the section 10.1 of the CSS 2.1 specification ([CSS21]) for details.
As a result of the computation of containing blocks for the tree, the boxes belonging to the wrapping contexts of all the elements can be determined:
e2
box: WC-1 (Wrapping
Context 1)
d1
inherits the body element's wrapping context and adds the e1-box
to it. So the wrapping context is made of both
the e1-box
and the e2-box
:
WC-2
d2
inherits the body element's wrapping context: WC-1
In this step, each containing block is processed in turn. For each containing block, we (conceptually) go through two phases:
In our example, this breaks down to:
d1
element's wrapping context: RWC-2
d1
element
d2
element's wrapping context: RWC-1
d2
element
The top-most wrapping context in the
layout tree contains the e2
exclusion. Its position
and size needs to be resolved. In general, computing an exclusion's
position and size may or may not require laying out other content. In our
example, no content needs to be laid out to resolve the e2
exclusion's position because it is absolutely
positioned and its size can be resolved without layout either. At this
point, RWC-1 is resolved and can be used when laying inline content out.
The process is similar: the position of the e1
exclusion needs to be resolved. Again, resolving the exclusion's position
and size may require processing the containing block (d1 here). It is the
case here because the size and position of e1
depend on resolving the percentage lengths. The percentages are relative
to the size of d1
‘s box. As a
result, in order to resolve a size for
’s box, a first layout of d1
d1
is done without any wrapping context (i.e., no exclusions
applied). The layout yields a position and size for e1
‘s box.
At this point, RWC-2 is resolved because the position and size of both e1 and e2 are resolved.
The important aspect of the above processing example is that once an element’s wrapping context is resolved (by resolving its exclusions' position and size), the position and size of the exclusions are not re-processed if the element's size changes between the layout that may be done without considering any wrapping context (as for RWC-2) and the layout done with the resolved wrapping context. This is what breaks the possible circular dependency between the resolution of wrapping contexts and the layout of containing blocks.
There are similarities between floats and exclusions in that inline content wraps around floats and also wraps around exclusion areas. However, there are very significant differences.
float
’ on the line box. Authors can
control how the floats move on the line box, to the right or to the left.
By contrast, exclusions can be positioned using any positioning scheme
such as grid layout ([CSS3-GRID-LAYOUT]),
flexible box ([CSS3-FLEXBOX]) or any other
CSS positioning scheme.
Floats have an effect on the positioning of exclusions and the layout of their inline content. For example, if an exclusion is an inline-box which happens to be on the same line as a float, its' position, as computed in Step 2-A will be impacted by the float, as is any other inline content.
Exclusions have an effect on the positioning of floats as they have an effect on inline content. Therefore, in Step 2-B, floats will avoid exclusion areas.
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"
, like this:
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.
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 be ignored.
To avoid clashes with future CSS features, the CSS 2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.
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 Working Group.
Further information on submitting testcases and implementation reports can be found from on the CSS Working Group's website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.
This specification is made possible by input from Andrei Bucur, Alexandru Chiculita, Arron Eicholz, Daniel Glazman, Arno Gourdol, Chris Jones, Bem Jones-Bey, Marcus Mielke, Alex Mogilevsky, Hans Muller, Mihnea Ovidenie, Virgil Palanciuc, Peter Sorotokin, Bear Travis, Eugene Veselov, Stephen Zilles and the CSS Working Group members.
Property | Values | Initial | Applies to | Inh. | Percentages | Media |
---|---|---|---|---|---|---|
wrap-flow | auto | both | start | end | minimum | maximum | clear | auto | block-level elements. | no | N/A | visual |
wrap-through | wrap | none | wrap | block-level elements | no | N/A | visual |