CSS Exclusions define arbitrary areas around which inline content ([[!CSS21]]) content 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.
CSS Shapes control the geometric shapes used for wrapping inline flow content outside or inside an element. CSS Shapes can be applied to any element. A circle shape on a float will cause inline content to wrap around the circle shape instead of the float's bounding box.
Combining CSS Exclusions and CSS Shapes allows sophisticated layouts, allowing interactions between shapes in complex positioning schemes.
This section is not normative.
The exclusions section of this specification defines features that allow inline flow content to wrap around outside the exclusion area of elements.
The shapes section of the specification defines properties to control the geometry of an element's exclusion area as well as the geometry used for wrapping an element's inline flow content.
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 areaThe area used for excluding inline flow content around an exclusion box. The exclusion area is equivalent to the border box for an exclusion box. This specification's 'shape-outside' property can be used to define arbitrary, non-rectangular exclusion areas. The 'shape-inside' property also defines an exclusion area, but in this case it is the area outside the shape that inline content avoids.
Float areaThe area used for excluding inline flow content around a float element. By default, the float area is the float element's margin box. This specification's 'shape-outside' property can be used to define arbitrary, non-rectangular float areas.
Exclusion elementAn 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 contextshould 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 and elements with 'shape-inside'. 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 areaThe content area is normally used for layout of the inline flow content of a box.
Wrapping areaThe 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
shrink-to-fit circle / shape
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 shape and the impact of other exclusions on other exclusions is controlled by the 'z-index' property as explained in the exclusions order section.
The shape properties can be used to change the shape of exclusion areas.An element becomes an exclusion when its 'wrap-flow' property has a computed value other than 'auto'.
Name: | wrap-flow |
---|---|
Value: | auto | both | start | end | minimum | maximum | clear |
Initial: | auto |
Applies to: | block-level elements. |
Inherited: | no |
Percentages: | N/A |
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 shape is contributed to its containing block's wrapping context, causing the containing block's descendants to wrap around its exclusion area.
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.
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.
Fluidity of the layout with respect to different amounts of content
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.
Name: | wrap-through |
---|---|
Value: | wrap | none |
Initial: | wrap |
Applies to: | block-level elements |
Inherited: | no |
Percentages: | N/A |
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.
Improve Example 3 about exclusion order
Ordering of exclusions.
<style type="text/css">
.exclusion {
wrap-flow: both;
position: absolute;
width: 50%;
height: auto;
}
</style>
<div class=”exclusion” style=”top: 0px; left: 0px;”>
Lorem ipsum dolor sit amet...
</div>
<div id="orderedExclusion" class=”exclusion” style=”top: 25%; left: 25%;”>
Lorem ipsum dolor sit amet...
</div>
<div class=”exclusion” style=”top: 50%; left: 50%;”>
Lorem ipsum dolor sit amet...
</div>
#orderedExclusion{ z-index: auto; } |
#orderedExclusion{ 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 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 (which may be different from the exclusion box because of the 'shape-outside' property).
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:
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-2d2
inherits the body element's
wrapping context: WC-1In 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-2d1
elementd2
element's wrapping context: RWC-1d2
elementThe 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 d1
's box, a first layout of 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.
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.
Handling visible content as a shape for Exclusions
Shapes define arbitrary geometric contours around which or into which inline flow content flows. There are two different types of shapes – 'outside' and 'inside'. The outside shape defines the exclusion area for an exclusion element or the float area for a float. The inside shape defines an element's content shape and the element's inline content will flow within that shape.
It is important to note that while outside shapes only apply to exclusions and floats, inside shapes apply to all block-level elements.
While the boundaries used for wrapping inline flow content outside and inside an element can be defined using shapes, the actual box model does not change. If the element has specified margins, borders or paddings they will be computed and rendered according to the [[!CSS3BOX]] module.
However, floats are an exception. If a float has an outside shape, its positioning is resolved as defined in [[!CSS21]] but using the outside shape's bounding box is used in lieu of the float's margin box.
CSS 'shape-outside' and CSS box model relation: the red box illustrates the exclusion element's content box, which is unmodified and subject to normal CSS positioning (here absolute positioning).
<style type="text/css">
.exclusion {
wrap-flow: both;
position: absolute;
top: 25%;
left: 25%;
width: 50%;
height: 50%;
shape-outside: circle(50%, 50%, 50%);
border: 1px solid red;
}
</style>
<div style=”position: relative;”>
<div class=”exclusion”></div>
Lorem ipsum dolor sit amet...
</div>
In the following example the left and right floating div
elements
specify a triangular shape using the 'shape-outside' property.
<div style="text-align:center;">
<div id="float-left"></div>
<div id="float-right"></div>
<div>
Sometimes a web page's text content appears to be
funneling your attention towards a spot on the page
to drive you to follow a particular link. Sometimes
you don't notice.
</div>
</div>
<style type="text/css">
#float-left {
shape-outside: polygon(0,0 100%,100% 0,100%);
float: left;
width: 40%;
height: 12ex;
}
#float-right {
shape-outside: polygon(100%,0 100%,100% 0,100%);
float: right;
width: 40%;
height: 12ex;
}
</style>
</div>
Shapes can be specified using
syntax similar to SVG's basic shapes.
The definitions use
<length>
type
and the <percentage>
types (see [[!CSS3VAL]]).
Percentages are resolved
from the computed value of the
'box-sizing'
property [[!CSS3UI]]
on the element to which the property applies.
For the radius r
of the circle
shape,
a percentage value is resolved as
specified
in the SVG recommendation (see [[!SVG11]]).
Path styling like stroking is not considered part of the specified shape.
The UA will close a polygon by connecting the last vertex with the first vertex of the list.
An SVG shape can be referenced using the url()
syntax. The shape can be
any of the SVG basic shapes or a
path element.
<style>
div {
height: 400px;
width: 400px;
}
.in-a-circle {
shape-inside: url(#circle_shape);
}
.in-a-path {
shape-inside: url(#path-shape);
}
</style>
<svg ...>
<circle id="circle_shape" cx="50%" cy="50%" r="50%" />
<path id="path-shape" d="M 100 100 L 300 100 L 200 300 z" />
</svg>
<div class="in-a-circle">...</div>
<div class="in-a-path">...</div>
When using the SVG syntax or referencing SVG elements to define shapes, the relevant box is determined by the computed value of the 'box-sizing' property. All the lengths expressed in percentages are resolved from the relevant box. The coordinate system for the shape has its origin on the top-left corner of the relevant box with the x-axis running to the right and the y-axis running downwards. If the SVG element uses unitless coordinate values, they are equivalent to using 'px' units. If the relevant box of the element is dependent on auto sizing (i.e., the element's 'width' or 'height' property is 'auto'), then the percentage values resolve to 0.
For interpolating between one basic shape and a second, the rules described below are applied.
Do we need to provide properties to repeat exclusion images as for the background-image property?
Use the contour keyword in shape-outside property?
Address security concern with automatic shape extractions for images
Another way of defining shapes is by specifying a source image whose alpha channel is used to compute the inside or outside shape. The shape is computed to be the path that encloses the area where the opacity of the specified image is greater than the 'shape-image-threshold' value. If the 'shape-image-threshold' is not specified, the initial value to be considered is 0.5.
Note, images can define cavities and inline flow content should wrap inside them. In order to avoid that, another exclusion element can be overlaid.
For animated raster image formats (such as GIF), the first frame of the animation sequence is used. For SVG images ([[SVG11]]), the image is rendered without animations applied.
An image is floating to the left of a paragraph. The image shows the 3D version of the CSS logo over a transparent background. The logo has a shadow using an alpha-channel.
The image defines its float area through the 'shape-outside' property and specifies a value of 35 pixels for the 'shape-margin' property.
<p>
<img id="CSSlogo" src="CSS-logo1s.png"/>
blah blah blah blah...
</p>
<style>
#CSSlogo {
float: left;
shape-outside: url("CSS-logo1s.png");
shape-image-threshold: 0.1;
shape-margin: 35px;
}
</style>
The image needs two references to the image because this example uses the same image
It is perfectly possible to display an image and use a different image for its float area.
In the figure below, the alpha-channel threshold is represented by the dotted line around the CSS logo and the 35px shape-margin is visible between that line and the edges of each individual line of the paragraph.
It's then possible to affect where the lines of the paragraph start in three ways:
Shapes are declared with the 'shape-outside' or 'shape-inside' properties, with possible modifications from the 'shape-margin' and 'shape-padding' properties. The shape defined by the 'shape-outside' and 'shape-margin' properties changes the geometry of an exclusion element's exclusion area or a float element's float area. If the element is not an exclusion element (see the 'wrap-flow' property) or a float, then the 'shape-outside' property has no effect.
The shape defined by the 'shape-inside' and 'shape-padding' properties defines an exclusion area that contributes to the element's wrapping context. The 'shape-inside' property applies to all block-level elements.
Name: | shape-outside |
---|---|
Value: | auto | <basic-shape> | <uri> |
Initial: | auto |
Applies to: | exclusion elements and floats |
Inherited: | no |
Percentages: | N/A |
Computed value: | computed lengths for <basic-shape>, the absolute URI for <uri>, otherwise as specified |
The values of this property have the following meanings:
The above figure shows how 'shape-outside' shapes impact the exclusion areas. The red box represents an element's content box and 'A', 'B', 'C' and 'C' represent exclusions with a complex shape and their 'wrap-flow' property computes to 'both', 'start', 'end' and 'clear', respectively.
As illustrated in the picture, when an exclusion allows wrapping on all sides, text can flow inside 'holes' in the exclusion (as for exclusion 'A'). Otherwise, the exclusion clears the area on the side(s) defined by 'wrap-flow', as illustrated for 'B', 'C' and 'D' above.
The 'shape-inside' property adds one or more exclusion areas to the element's wrapping context. This modifies the normal rectangular shape of the content area to a possibly non-rectangular wrapping area. The exclusion areas are defined by subtracting the shape from the element's content area. Any part of the shape outside the element's content area has no effect.
Name: | shape-inside |
---|---|
Value: | outside-shape | auto | <basic-shape> | <uri> |
Initial: | outside-shape |
Applies to: | block-level elements |
Inherited: | no |
Percentages: | N/A |
Computed value: | computed lengths for <basic-shape>, the absolute URI for <uri>, otherwise as specified |
Should we revisit the decision to not allow SVG path syntax in the shape-inside, shape-outside properties
The values of this property have the following meanings:
The 'shape-inside' property applies to floats.
The 'shape-inside' property may not apply on some elements such as elements with a computed 'display' value of 'table'.
Overflow content avoids the exclusion area(s) added by 'shape-inside' and 'shape-padding' (as well as any other exclusion areas in the element's wrapping context).
The 'shape-image-threshold' defines the alpha channel threshold used to extract the shape using an image. A value of 0.5 means that all the pixels that are more than 50% transparent define the path of the exclusion shape. The 'shape-image-threshold' applies to both 'shape-outside' and 'shape-inside'.
The specified value of 'shape-image-threshold' is applied to both images used for 'shape-outside' and 'shape-inside'.
Name: | shape-image-threshold |
---|---|
Value: | <alphavalue> |
Initial: | 0.5 |
Applies to: | block-level elements |
Inherited: | no |
Percentages: | alpha channel of the image specified by <uri> |
Computed value: | The same as the specified value after clipping the <alphavalue> to the range [0.0,1.0]. |
The values of this property have the following meanings:
The 'shape-margin' property adds a margin to a shape-outside. This defines a new shape where every point is the specified distance from the shape-outside. This property takes on positive values only.
Name: | shape-margin |
---|---|
Value: | <length> |
Initial: | 0 |
Applies to: | exclusion elements and floats |
Inherited: | no |
Percentages: | N/A |
Computed value: | the absolute length |
A 'shape-margin' creating an offset from a circular shape-outside. The blue rectangles represent inline content affected by the shape created by the margin.
The 'shape-padding' property adds padding to a shape-inside. This defines a new shape where every point is the specified distance from the shape-inside. This property takes on positive values only.
Name: | shape-padding |
---|---|
Value: | <length> |
Initial: | 0 |
Applies to: | block-level elements |
Inherited: | no |
Percentages: | N/A |
Computed value: | the absolute length |
A 'shape-padding' creating an offset from a circular shape-inside. The light blue rectangles represent inline content affected by the shape created by the padding.
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.