CSS Shapes Module Level 1

1. Introduction

This section is not normative.

Shapes define arbitrary geometries that can be used as CSS values. This specification defines properties to control the geometry of an element’s float area. The shape-outside property uses shape values to define the float area for a float.

Note: Future levels of CSS Shapes will allow use of shapes on elements other than floats. Other CSS modules can make use of shapes as well, such as CSS Masking [CSS-MASKING] and CSS Exclusions [CSS3-EXCLUSIONS].

Note: If a user agent implements both CSS Shapes and CSS Exclusions, the shape-outside property defines the exclusion area for an exclusion.

Note: A future level of CSS Shapes will define a shape-inside property, which will define a shape to wrap content within the element.

1.1. Module Interactions

This module extends the float features defined in [CSS2] chapter 9.

1.2. Values

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

1.3. Terminology

Wrap

This specification uses the term wrap to refer to flowing content around the sides of a float area, defined in [CSS2] chapter 9. Content wraps around the right side of a left-floated box, and content wraps around the left side of a right-floated box. One result of this wrapping is that line boxes next to a float are shortened as necessary to avoid intersections with the float area.

Float area

The area used for wrapping content around a float element. The rules for float behavior use the sides of the float area to determine where content flows. By default, the float area is the float element’s margin box (note this can be different than the float area produced by the margin-box value, which includes border-radius curvature). This specification’s shape-outside and shape-margin properties can be used to define an arbitrary, non-rectangular float area.

direction-agnostic size The direction-agnostic size of a box is equal to the length of the diagonal of the box, divided by sqrt(2).

Note: This is a method of averaging the width and height of a box used by SVG in some cases, when a percentage of a box’s size is desired but the context doesn’t specifically favor the width or the height. For square boxes, this is the same as the width/height.

2. Relation to the box model and float behavior

While the boundaries used for wrapping inline flow content outside a float can be defined using shapes, the actual box model does not change. If the element has specified margins, borders or padding they will be computed and rendered according to the [CSS3BOX] module. Also, float positioning and stacking are not affected by defining a float area with a shape.

When a shape is used to define a float area, the shape is clipped to the float’s margin box. In other words, a shape can only ever reduce a float area, not increase it. A reduced float area may have no effect on some line boxes that would normally be affected by the float. If a shape does not enclose any area, the shape’s edges are still used to define the float area.

A float area defined by a shape may reduce the normal float area on all sides, but this does not allow content to wrap on both sides of a float. Left floats with a shape-outside still only allow content wrapping on the right side, and right floats only allow wrapping on the left.

In the following example the left and right floating img elements specify a triangular shape using the shape-outside property.

<img class="left" src="hand.svg"/>
<img class="right" src="hand.svg"/>
<p>
  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.
</p>

<style type="text/css">
  .left {
    shape-outside: polygon(0 0, 100% 100%, 0 100%);
    float: left;
    width: 40%;
    height: 12ex;
    transform: scaleX(-1);
  }

  .right {
    shape-outside: polygon(100% 0, 100% 100%, 0 100%);
    float: right;
    width: 40%;
    height: 12ex;
  }

  p {
    text-align: center;
  }
</style>

Using the shape-outside property with floats

Since shapes are clipped to the float’s margin box, adding this shape to the left float above would result in the same rendering.

shape-outside: polygon(0 0, 500% 500%, 0 500%);

A shape that does not enclose any area still has edges that contribute to the float area.

This inset shape is a vertical line positioned at the midpoint of the reference box. This midpoint edge is used as the edge of the float area for wrapping content.

shape-outside: inset(0% 50% 0% 50%);

If inset values add up to more than the width, CSS Backgrounds 3 § 4.5 Overlapping Curves rules are used to determine the edges of the rectangle. This shape results in a vertical edge 25% from the left side of the reference box.

shape-outside: inset(0% 150% 50% 0%);

If the shape is only a horizontal line, then it is an empty float area and has no effect on wrapping. Note that in this example shape-margin must be 0px (otherwise the line would expand to enclose an area).

shape-outside: inset(50% 0% 0% 50%);
shape-margin: 0px;

A shape-outside can create open areas on both the left and right of a float area. Content still wraps only on one side of a float in this case. In the picture, the shape is rendered in blue, and the content area outside the shape in mauve.

shape-outside: polygon(50px 0px, 100px 100px, 0px 100px);

wrapping around right side of a left-float float area

The following styling creates a shape much smaller than the float’s content area, and adds a margin-top to the float. In the picture, the shape is rendered in blue, the content area outside the shape in mauve, and the margin area of the float box in yellow. The inline content only wraps around the shape, and otherwise overlays the rest of the float margin box.

.float-left {
  shape-outside: polygon(0% 50%, 50% 100%, 0 100%);
  float: left;
  width: 100px;
  height: 100px;
  margin-top: 20px;
}

Adding margin-top to a float with a small shape-outside

The next picture shows a possible result if two of these floats were stacked next to each other. Note that the floats are positioned using their margin boxes, not the float area.

Stacking two floats with a small shape-outside

3. Basic Shapes

The <basic-shape> type can be specified using basic shape functions. When using this syntax to define shapes, the reference box is defined by each property that uses <basic-shape> values. The coordinate system for the shape has its origin on the top-left corner of the reference box with the x-axis running to the right and the y-axis running downwards. All the lengths expressed in percentages are resolved from the used dimensions of the reference box.

3.1. Supported Shapes

The <basic-shape> functions are:

<inset()> = inset(
  <length-percentage>{1,4}
  [ round <'border-radius'> ]?
)

<xywh()> = xywh(
  <length-percentage>{2} <length-percentage [0,∞]>{2}
  [ round <'border-radius'> ]?
)

<rect()> = rect(
  [ <length-percentage> | auto ]{4}
  [ round <'border-radius'> ]?
)

<basic-shape-rect> = <inset()> | <rect()> | <xywh()>

<circle()> = circle(
  <radial-size>?
  [ at <position> ]?
)

<ellipse()> = ellipse(
  <radial-size>?
  [ at <position> ]?
)

<polygon()> = polygon(
  <'fill-rule'>? 
  [ round <length> ]? ,
  [<length-percentage> <length-percentage>]#
)

<path()> = path(
  <'fill-rule'>? ,
  <string>
)

<shape()> = shape(
  <'fill-rule'>?
  from <position>
  <shape-command>#
)

inset()

Defines an inset rectangle via insets from each edge of the reference box.

If less than four <length-percentage> values are provided, the omitted values default in the same way as the margin shorthand: an omitted second or third value defaults to the first, and an omitted fourth value defaults to the second.

The four <length-percentage>s define the position of the top, right, bottom, and left edges of a rectangle, respectively, as insets from the corresponding edges of the reference box.

A pair of insets in either dimension that add up to more than the used dimension (such as left and right insets of 75% apiece) use the CSS Backgrounds 3 § 4.5 Overlapping Curves rules to proportionally reduce the inset effect to 100%.

For example, specifying inset(75% 0 50% 0) has the top+bottom edges summing to 125% of the reference box’s height. They’re proportionally reduced to sum to 100%, identical to specifying inset(60% 0 40% 0).

The optional <'border-radius'> argument(s) define rounded corners for the rectangle using the border-radius shorthand syntax.

Tests

xywh()

Defines a rectangle via offsets from the top and left edge of the reference box, and a specified width and height.

The four <length-percentage>s define, respectively, the inset from the left edge of the reference box, the inset from the top edge of the reference box, the width of the rectangle, and the height of the rectangle.

Note: This syntax is inspired by the viewBox attribute from SVG.

The optional <'border-radius'> argument(s) define rounded corners for the inset rectangle using the border-radius shorthand syntax.

Tests

rect() =

Defines a rectangle via insets from the top and left edges of the reference box.

The four <length-percentage>s define the position of the top, right, bottom, and left edges of a rectangle, respectively, as insets from the top edge of the reference box (for the first and third values) or the left edge of the reference box (for the second and fourth values).

An auto value makes the edge of the box coincide with the corresponding edge of the reference box: it’s equivalent to 0% as the first (top) or fourth (left) value, and equivalent to 100% as the second (right) or third (bottom) value.

The second (right) and third (bottom) values are floored by the fourth (left) and second (top) values, respectively.

For example, specifying rect(10px 0 0 20px) would place the bottom edge higher than the top edge, and the right edge further left than the left edge, so both are corrected to not cross over the other edge, identical to specifying rect(10px 20px 10px 20px).

Note: This syntax is similar, but not quite identical, to the legacy rect() function used solely by the clip property.

The optional <'border-radius'> argument(s) define rounded corners for the rectangle using the border-radius shorthand syntax.)

Tests

circle()

The <radial-size> argument defines the circle’s radius. Rather than referring to the gradient box, values are resolved against the reference box.
Two <length-percentage> values are invalid.

The <position> argument defines the center of the circle. Unless otherwise specified, this defaults to center if omitted.

Tests

ellipse()

The <radial-size> argument defines the horizontal and vertical radiuses of the ellipse. Rather than referring to the gradient box, values are resolved against the reference box.
The <position> argument defines the center of the ellipse. Unless otherwise specified, this defaults to center if omitted.

Tests

polygon()

The <'fill-rule'> specifies the filling rule used to determine the interior. Defaults to nonzero if omitted.
An optional <length> after a round keyword defines rounding for each vertex of the polygon. The length is the radius of a circle whose center lies on the bisector of the smaller angle of the vertex, and that is tangential to both sides of the vertex.

Rounding polygon vertices that are both convex and concave.

To avoid rounding more than half of any line segment, the rounding of each vertex must be clamped separately such that the radius is never more than the smaller of tan(angle/2) segment / 2 evaluated against both vertex line segments.

This diagram shows the intent of the clamping formula.
Each <length-percentage> pair specifies a vertex of the polygon, as a horizontal and vertical offset from the left and top edges of the reference box.

The UA must close a polygon by connecting the last vertex with the first vertex of the list.

Tests

path()

The <'fill-rule'> specifies the filling rule used to determine the interior. Defaults to nonzero if omitted, unless the function is being used in a context such as SVG shapes where the fill-rule property is relevant. In that case an omitted value will use the computed value of the fill-rule property.
The <string> represents an SVG Path data string. A path data string that does not conform to the to the grammar and parsing rules of SVG 1.1, or that does conform but defines an empty path, is invalid and causes the entire path() to be invalid.

The initial position is defined by the first “move to” argument in the path string. For the initial direction follow SVG 1.1.

The UA must close a path with an implicit closepath command ("z" or "Z") if it is not present in the string for properties that require a closed loop (such as shape-outside and clip-path).

Tests

shape()

See The shape() function.

Tests

3.1.1. The shape() Function

While the path() function allows reuse of the SVG path syntax to define more arbitrary shapes than allowed by more specialized shape functions, it requires writing a path as a single string (which is not compatible with, for example, building a path piecemeal with var()), and inherits a number of limitations from SVG, such as implicitly only allowing the px unit.

The shape() function uses a set of commands roughly equivalent to the ones used by path(), but does so with more standard CSS syntax, and allows the full range of CSS functionality, such as additional units and math functions. The commands used by shape() are dynamically turned into path segments when it is used for rendering, e.g., when computing the rendered clip-path.

In that sense, shape() is a superset of path(). A path() can be easily converted to a shape(), but to convert a shape() back to a path() or to SVG requires information about the CSS environment (e.g. current values of CSS custom properties, current font size for em units, etc).

The <'fill-rule'> is interpreted identically to the same argument in path().

The rest of the arguments define a list of path data commands, identical to that of an SVG Path, which the function represents.

The from <coordinate-pair> represents the starting point for the first shape-command. It adds an initial absolute moveto to the list of path data commands.

The sequence of <shape-command>s represent further path data commands. Each command’s starting point is the previous command’s ending point.

<shape-command> = <move-command> | <line-command> | close |
                  <horizontal-line-command> | <vertical-line-command> |
                  <curve-command> | <smooth-command> | <arc-command>

<move-command> = move <command-end-point>
<line-command> = line <command-end-point>
<horizontal-line-command> = hline
        [ to [ <length-percentage> | left | center | right | x-start | x-end ]
        | by <length-percentage> ]
<vertical-line-command> = vline
        [ to [ <length-percentage> | top | center | bottom | y-start | y-end ]
        | by <length-percentage> ]
<curve-command> = curve
        [ [ to <position> with <control-point> [ / <control-point> ]? ]
        | [ by <coordinate-pair> with <relative-control-point> [ / <relative-control-point> ]? ] ]
<smooth-command> = smooth
        [ [ to <position> [ with <control-point> ]? ]
        | [ by <coordinate-pair> [ with <relative-control-point> ]? ] ]
<arc-command> = arc <command-end-point>
            [ [ of <length-percentage>{1,2} ]
              && <arc-sweep>? && <arc-size>? && [rotate <angle>]? ]

<command-end-point> = [ to <position> | by <coordinate-pair> ]
<control-point> = [ <position> | <relative-control-point> ]
<relative-control-point> = <coordinate-pair> [ from [ start | end | origin ] ]?
<coordinate-pair> = <length-percentage>{2}
<arc-sweep> = cw | ccw
<arc-size> = large | small

<coordinate-pair> = <length-percentage>{2}

Defines a pair of coordinates, representing a rightward and downward offset, respectively, from a specified reference point. Percentages are resolved against the width or height, respectively, of the reference box.

<command-end-point> = [ to <position> | by <coordinate-pair> ]

Every command can be specified in "absolute" or "relative" coordinates, determined by their by or to component. to indicates that any <coordinate-pair>s in the command are relative to the top-left corner of the reference box, while by indicates that the <coordinate-pair>s are relative to the command’s starting point.

<relative-control-point> defines how by and to are interpreted for curve control points, while <horizontal-line-command> and <vertical-line-command> define how by and to are interpreted for horizontal and vertical lines, respectively.

When to is used, the coordinates can be specified as <position>s instead of <coordinate-pair>s.

Note: In either case, <percentage> values in <coordinate-pair>s are always computed relative to the reference box’s size.

<move-command> = move <command-end-point>

Adds a moveto command to the list of path data commands, with an ending point specified by the <coordinate-pair>.

This draws nothing, and merely "moves the pen" for the next command.

Note: This starts a new subpath, for the purpose of the close command.

<line-command> = line <command-end-point>

Adds a lineto command to the list of path data commands, with an ending point specified by the <coordinate-pair>.

This draws a straight line from the command’s starting point to its ending point.

Adds a horizontal lineto command to the list of path data commands.

This is equivalent to a line command with the <length-percentage> given as the horizontal component of the <coordinate-pair>. Specifying the horizontal component of <position> instead of a <length-percentage> (left, center, right, x-start, or x-end), would draw a line to that <position>, with the <position>’s vertical component remaining the same as the starting point.

Adds a vertical lineto command to the list of path data commands.

This is equivalent to a line command with the <length-percentage> given as the vertical component of the <coordinate-pair>. Specifying the horizontal component of <position> (top, center, bottom, y-start, or y-end) instead of a <length-percentage>, would draw a line to that <position>, with the <position>’s horizontal component remaining the same as the starting point.

<curve-command> = curve [ [ to <position> with <control-point> [ / <control-point> ]? ] | [ by <coordinate-pair> with <relative-control-point> [ / <relative-control-point> ]? ] ]

Adds a Bézier curve command to the list of path data commands, ending at the point specified by the <position> following the to keyword, or the <coordinate-pair> following the by keyword, as specified by <command-end-point>.

The with component specifies control points for the curve: if a single <control-point> or <relative-control-point> is provided, the command specifies a quadratic curve; if two <control-point>s or <relative-control-point>s are provided, it specifies a cubic curve.

<smooth-command> = smooth [ [ to <position> [with <control-point> ]? ] | [ by <coordinate-pair> [ with <relative-control-point> ]? ] ]

Adds a smooth Bézier curve command to the list of path data commands, ending at the point specified by the <position> following the to keyword, or the <coordinate-pair> following the by keyword, as specified by <command-end-point>. The with component specifies control points for the curve: if it’s omitted, the command specifies a smooth quadratic curve; if it’s provided, if specifies a smooth cubic curve.

Note: A smooth command is equivalent to a curve command with the first control point automatically specified as the reflection of the previous curve’s second control point around the starting point, or as the starting point if the previous path data command wasn’t a curve. This ensures G1 continuity with the previous command, so the curve appears to smoothly continue from the previous command, rather than possibly making a sudden direction change.

<control-point> = [ <position> | <relative-control-point> ]

Provides a control point to a quadratic or cubic Bézier curve.

<relative-control-point> = <coordinate-pair> [ from [ start | end | origin ] ]?

Provides a control point to a quadratic or cubic Bézier curve. When a from keyword is specified followed by start, end, or origin, the given <coordinate-pair> is relative to the command’s starting point, the command’s end point, or the reference box, respectively. If such component is not provided, the <coordinate-pair> is relative to the segment’s start.

<arc-command> = arc <command-end-point> [[of <length-percentage>{1,2}] && <arc-sweep>? && <arc-size>? && rotate <angle>? ]

Add an elliptical arc command to the list of path data commands, ending at the <command-end-point>.

The of component specifies the size of the ellipse that the arc is taken from. The first <length-percentage> provides the horizontal radius of the ellipse and the second provides the vertical radius. Like for <coordinate-pair>s, <percentage> values are resolved against the width or height of the reference box, as appropriate.

If only one <length-percentage> is provided, both radiuses use the provided value. In that case, <percentage> values are resolved against the direction-agnostic size of the reference box (similar to the circle() function).

Note that SVG has some specific error-handling for the ellipse radiuses:

if the endpoint is the same as the starting point, the command does nothing
if either radius is zero, the command is equivalent to a <line-command> to the ending point
if either radius is negative, its absolute value is used instead
if the radiuses don’t describe an ellipse large enough to intersect both the starting point and ending point (after rotation by the specified <angle>), they are scaled up uniformly until the ellipse is just large enough to reach.

The ellipse described by the specified radiuses defaults to being axis-aligned, but can be rotated by specifying an <angle>. Similar to the rotate() transform function, positive angles specify a clockwise rotation, and negative angles specify a counterclockwise rotation. If omitted, this defaults to 0deg.

The ending point, radiuses, and angle, taken together, usually define two possible ellipses that intersect the starting point and ending point, and each ellipse can be traced in either direction, for a total of four possible arcs. The <arc-sweep> and <arc-size> components specify which of these arcs is desired:

<arc-sweep> can be cw or ccw, indicating that the arc that is traced around the ellipse clockwise or counter-clockwise from the center, respectively, must be chosen. If omitted, this defaults to ccw.

Note: In the SVG arc command, cw corresponds to the value 1 for the sweep flag, and ccw to the value 0.
<arc-size> can be large or small, indicating that the larger or smaller, respectively, of the two possible arcs must be chosen. If omitted, this defaults to small.

Note: In the SVG arc command, large corresponds to the value 1 for the large flag, and small to the 0.

Note: If the starting and ending points are on exactly opposite sides of the ellipse, both possible arcs are the same size, but also there is only one possible ellipse. In this case, the <arc-sweep> distinguishes which of the two possible arcs will be chosen, and <arc-size> has no effect.

A depiction of the two possible ellipses, and four possible arcs, that can be chosen between.

close

Adds a closepath command to the list of path data commands.

Note: This is similar to a line command with its ending point set to the starting point of the subpath. When specifying a raw shape, they’re identical, but if the path is stroked, the ending point of the close command is smoothly joined with the start of the subpath, which affects how line-joins and line-caps are rendered.

3.1.1.1. Using shape() to create responsive, parametric speech bubble

The shape() function enables shapes that are responsive, rather than scalable. While the polygon() shape is also responsive, it only support simple rounded corners and not complex curves.

To demonstrate, let’s start with a speech bubble, such as the following:

Using this shape with a clip-path can be done by using the path() function:

.bubble { clip-path: path("m 5 0 H 95 Q 100 0 100 5 V 92 Q 100 97 95 97 H 70 l -2 3 l -2 -3 H 5 Q 0 97 0 92 V 5 Q 0 0 5 0") };

Altohugh this path can easily scale, the scaled results are not always desirable. e.g. when scaled to a small balloon, the arrow and corners are scaled to become almost invisible:

To construct this shape using the shape() function, let’s start by turning all the pixel values from the path function to percentages. Note that the shape() function begins with from:

.bubble { clip-path: shape(  from 5% 0%,
              hline to 95%,
              curve to 100% 5% with 100% 0%,
              vline to 92%,
              curve to 95% 97% with 100% 97%,
              hline to 70%,
              line by -2% 3%,
              line by -2% -3%,
              hline to 5%,
              curve to 0% 92% with 0% 97%,
              vline to 5%,
              curve to 5% 0% with 0% 0%); }

To make this path responsive, as in, respond well to size changes, we will convert some of its units to px values, specifically the ones the control the curves and arrows:

.bubble { clip-path: shape(  from 5px 0%,
              hline to calc(100% - 5px),
              curve to 100% 5px with 100% 0%,
              vline to calc(100% - 8px),
              curve to calc(100% - 5px) calc(100% - 3px) with 100% calc(100% - 3px),
              hline to 70%,
              line by -2px 3px,
              line by -2px -3px,
              hline to 5px,
              curve to 0% calc(100% - 8px) with 0% calc(100% - 3px),
              vline to 5px,
              curve to 5px 0% with 0% 0%); }

When applied as clip-path, it would looks like the following:

The whole speech bubble is scaled to the reference box, while the curves and arrows stay more constant.

Since shape() uses CSS units, we can replace some of the edges with position values:

.bubble { clip-path: shape(from 5px 0,
              hline to calc(100% - 5px),
              curve to right 5px with right top,
              vline to calc(100% - 8px),
              curve to calc(100% - 5px) calc(100% - 3px) with right calc(100% - 3px),
              hline to 70%,
              line by -2px 3px,
              line by -2px -3px,
              hline to 5px,
              curve to left calc(100% - 8px) with left calc(100% - 3px),
              vline to 5px,
              curve to 5px top with left top); }

Another useful feature of shape() is that it can be used alongside CSS properties. In this case, we can make the arrow and radius parametric:

:root {
  --radius: 5px;
  --arrow-length: 3px;
  --arrow-half-width: 2px;
  --arrow-position: 70%;
  --arrow-bottom-offset: calc(100% - var(--radius) - var(--arrow-length));
}

.bubble {
  animation: bubble 100ms;
  clip-path: shape(from var(---radius) top,
    hline to calc(100% - var(---radius)),
    curve to right var(---radius) with right top,
    vline to var(---arrow-bottom-offset),
    curve to calc(100% - var(---radius)) calc(100% - var(---arrow-length))
              with right calc(100% - var(---arrow-length)),
    hline to var(---arrow-position),
    line by var(---arrow-half-width) var(---arrow-length),
    line by var(---arrow-half-width) calc(0px - var(---arrow-length)),
    hline to var(---radius),
    curve to left var(---arrow-bottom-offset) with left calc(100% - var(---arrow-length)),
    vline to var(---radius),
    curve to var(---radius) top with left top); }

3.1.1.2. Interpolating the shape() Function

shape() and path() functions can be interpolated with each other if their associated list of path data commands is the same length and has the same commands, in order, with the first command of the path() function interpolating with the initial <coordinate-pair> in the shape() function.

Note: The first command of a path() function is guaranteed to be a move, see moveTo in the SVG spec.

If the starting and ending values are both path() functions, the interpolated value is a path() function; otherwise it’s a shape() function. In either case, the interpolated value must represent the same list of path data commands, with each command having its numerical components interpolated between the corresponding components of the starting and ending list.

For this purpose, commands are "the same" if they use the same command keyword, and use the same <by-to> keyword. For curve and smooth, they also must have the same number of control points.

If an arc command has different <arc-sweep> between its starting and ending list, then the interpolated result uses cw for any progress value between 0 and 1. If it has different <arc-size> keywords, then the interpolated result uses large for any progress value between 0 and 1.

Note: The arc keyword interpolation rules are meant to match existing SVG path interpolation rules.

3.2. Computed Values of Basic Shapes

The values in a <basic-shape> function are computed as specified, with these exceptions:

Omitted values are included and compute to their defaults.
A <position> value in circle() or ellipse() is computed as a pair of offsets (horizontal then vertical) from the top left origin, each given as a <length-percentage>.
A <'border-radius'> value in a <basic-shape-rect> function is computed as an expanded list of all eight <length-percentage> values.
All <basic-shape-rect> functions compute to the equivalent inset() function.
Note: Given rect(t r b l), the equivalent function is inset(t calc(100% - r) calc(100% - b) l). Given xywh(x y w h), the equivalent function is inset(y calc(100% - x - w) calc(100% - y - h) x).

Tests

3.3. Serialization of Basic Shapes

To serialize the <basic-shape> functions, serialize as per their individual grammars, in the order the grammars are written in, joining space-separated tokens with a single space, and following each serialized comma with a single space. For serializing computed values, component values are computed, and omitted when possible without changing the meaning.

As specified value serialization of the shape functions are relatively trivial, here are some examples of computed value serializations for circle() notations when used in shape-outside:

<position> serialization rules mean that keywords compute to percentages, and serialize in horizontal-vertical order.
```
circle(at bottom left)
/* serializes to */
circle(at 0% 100%)
```
Omitting optional components means that default values do not show up in the serialization.
```
circle(closest-side at center)
/* serializes to */
circle()
```

Value computation means that some functions canonicalize to a different form.

rect(10px 20px 30px 40px)
/* serializes to */
inset(10px calc(100% - 20px) calc(100% - 30px) 40px)

Tests

basic-shape-circle-ellipse-serialization.html (live test) (source)

3.4. Interpolation of Basic Shapes

For interpolating between one basic shape and a second, the rules below are applied. The values in the shape functions interpolate by computed value. The list values interpolate as length, percentage, or calc where possible. If list values are not one of those types but are identical (such as finding nonzero in the same list position in both lists) those values do interpolate.

Both shapes must use the same reference box.
If both shapes are the same type, that type is ellipse() or circle(), and the radiuses are specified as <length-percentage> (rather than keywords), interpolate between each value in the shape functions.
If both shapes are of type inset(), interpolate between each value in the shape functions.
If both shapes are of type polygon(), both polygons have the same number of vertices, and use the same <'fill-rule'>, interpolate between each value in the shape functions.
In all other cases no interpolation is specified.

Tests

4. Shapes from Image

Another way of defining shapes is by specifying a source <image> whose alpha channel is used to compute the shape. The shape is computed to be the path or paths that enclose the area(s) where the opacity of the specified image is greater than the shape-image-threshold value. The absence of any pixels with an alpha value greater than the specified threshold results in an empty float area that will not affect wrapping. If the shape-image-threshold is not specified, the initial value to be considered is 0.0.

The image is sized and positioned as if it were a replaced element whose specified width and height are the same as the element’s used content box size.

For animated raster image formats (such as GIF), the first frame of the animation sequence is used.

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.


  <p>
    <img id="CSSlogo" src="CSS-logo1s.png"/>
    blah blah blah blah...
  </p>

  <style>
    #CSSlogo {
      float: left;
      shape-outside: attr(src url);
      shape-image-threshold: 0.1;
    }
  </style>

The shape-outside property re-uses the url from the src attribute of the img element.

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.

It’s then possible to affect where the lines of the paragraph start in three ways:

Modifying the alpha channel in the image
Changing the value of the shape-image-threshold property
Changing the value of the shape-margin property (see example 8)

A float shape around an image using its alpha-channel.

Tests

5. Shapes from Box Values

Shapes can be defined by reference to edges in the CSS Box Model. These edges include border-radius curvature [CSS3BG] from the used border-radius values. The <shape-box> value extends the <visual-box> value to include margin-box. Its syntax is:

<shape-box> = <visual-box> | margin-box

The definitions of the values are:

The margin-box value defines the shape enclosed by the outside margin edge. The corner radii of this shape are determined by the corresponding border-radius and margin values. If the ratio of border-radius/margin is 1 or more, or margin is negative or zero, then the margin box corner radius is max(border-radius + margin, 0). If the ratio of border-radius/margin is less than 1, and margin is positive, then the margin box corner radius is border-radius + margin * (1 + (ratio-1)^3).

The border-box value defines the shape enclosed by the outside border edge. This shape follows all of the normal border radius shaping rules for the outside of the border.

The padding-box value defines the shape enclosed by the outside padding edge. This shape follows all of the normal border radius shaping rules for the inside of the border.

The content-box value defines the shape enclosed by the outside content edge. Each corner radius of this box is the larger of 0 or border-radius - border-width - padding.

Given the 100px square below with 10px padding, border and margins, the box values define these shapes:

margin-box: the shape containing all of the yellow pixels
border-box: the shape containing all of the black pixels
padding-box: the shape containing all of the mauve pixels
content-box: the shape containing all of the blue pixels

Colored boxes representing simple box edges — Simple CSS Box Model Edges

And the same definitions apply to a more complex example with the same 100px square, but with these border, padding and margin properties:


  border-radius: 20px 20px 20px 40px;
  border-width: 30px 10px 20px 10px;
  padding: 10px 20px 10px 10px;
  margin: 20px 10px 10px 10px;

Colored boxes representing complex box edges — Complex CSS Box Model Edges

The difference between normal float wrapping and wrapping around the shape defined by the margin-box value is that the margin-box shape includes corner shaping. Take the 100px square with 10px padding, border and margins, but with a border-radius of 60px. If you make a left float out of it, content normally wraps in this manner:

Text wrapping around float with no shape — Normal float wrapping

If you add a margin-box shape to the float, then content wraps around the rounded margin-box corners.


  shape-outside: margin-box;

Text wrapping around float with margin-box shape — Float wrapping with margin-box

Tests

6. Declaring Shapes

Shapes are declared with the shape-outside property, with possible modifications from the shape-margin property. The shape defined by the shape-outside and shape-margin properties changes the geometry of a float element’s float area.

6.1. Float Area Shape: the shape-outside property

Name:	shape-outside
Value:	none \| [ <basic-shape> \|\| <shape-box> ] \| <image>
Initial:	none
Applies to:	floats and initial letter boxes
Inherited:	no
Percentages:	n/a
Computed value:	as defined for <basic-shape> (with <shape-box> following, if supplied); else the computed <image>; else the keyword as specified
Canonical order:	per grammar
Animation type:	as defined for <basic-shape>, otherwise discrete

The values of this property have the following meanings:

none: The float area is unaffected.
<shape-box>: If one of these values is specified by itself the shape is computed based on one of margin-box, border-box, padding-box or content-box which use their respective boxes including curvature from border-radius, similar to background-clip [CSS3BG].
<basic-shape>: The shape is computed based on one of the <basic-shape> functions. If a <shape-box> is also supplied, this defines the reference box for the <basic-shape> function. If <shape-box> is not supplied, then the reference box defaults to margin-box.
<image>: The shape is extracted and computed based on the alpha channel of the specified <image> as defined by shape-image-threshold.
User agents must use the CORS protocol defined by the [FETCH] specification for all URLs in a shape-outside value. When fetching, user agents must use "Anonymous" mode, set the referrer source to the stylesheet’s URL and set the origin to the URL of the containing document. If this results in network errors such that there is no valid fallback image, the effect is as if the value none had been specified.

Tests

6.2. Choosing Image Pixels: the shape-image-threshold property

The shape-image-threshold defines the alpha channel threshold used to extract the shape using an image. A value of 0.5 means that the shape will enclose all the pixels that are more than 50% opaque.

Name:	shape-image-threshold
Value:	<opacity-value>
Initial:	0
Applies to:	floats
Inherited:	no
Percentages:	n/a
Computed value:	specified number, clamped to the range [0,1]
Canonical order:	per grammar
Animation type:	by computed value

The values of this property have the following meanings:

<number>: Sets the threshold used for extracting a shape from an image. The shape is defined by the pixels whose alpha value is greater than the threshold. A threshold value outside the range 0.0 (fully transparent) to 1.0 (fully opaque) will be clamped to this range.

Note: A future level of CSS Shapes may define a switch to use the luminance data from an image instead of the alpha data. When this happens, shape-image-threshold will be extended to apply its threshold to either alpha or luminance, depending on the switch state.

Tests

6.3. Expanding a Shape: the shape-margin property

The shape-margin property adds a margin to a shape-outside. This defines a new shape that is the smallest contour (in the shrink-wrap sense) that includes all the points that are the shape-margin distance outward in the perpendicular direction from a point on the underlying shape. This includes any edge or line sections from the underlying shape. Note that at points where a perpendicular is not defined (e.g. sharp points or line ends) take all points on the circle centered at the point and with a radius of shape-margin.

The new shape produced by applying shape-outside is what determines the float area, and must be constructed before making any wrap decisions.

This property takes only non-negative values.

Name:	shape-margin
Value:	<length-percentage [0,∞]>
Initial:	0
Applies to:	floats and initial letter boxes
Inherited:	no
Percentages:	refer to the inline size of the containing block
Computed value:	computed <length-percentage> value
Canonical order:	per grammar
Animation type:	by computed value

<length-percentage [0,∞]>: Sets the margin of the shape to the specified value.

Note: Adding a shape-margin does NOT allow a float area to extend outside a float’s margin box. Extra margin may need to be applied along with shape-margin to avoid clipping.

A shape-margin creating an offset from a polygonal shape-outside. The lighter blue area shows the shape in a 100x100px float, and the darker blue area shows the 10px offset.


  .float {
      width: 100px;
      height: 100px;
    shape-outside: polygon(10px 10px, 90px 50px, 40px 50px, 90px 90px, 10px 90px);
    shape-margin: 10px;
  }

If shape-margin is added to the CSS logo from example 6, the line boxes wrapping around the shape are shortened further. In case the image’s alpha channel runs up to the right edge of the image, some extra margin-right should be applied to ensure the shape is not clipped by the margin box.


  #CSSlogo {
    shape-margin: 35px;
    margin-right: 35px;
  }

A float shape around an image using its alpha-channel with a 35 pixels shape-margin — A float shape around an image using its alpha-channel with a 35-pixel shape-margin

Tests

7. Privacy Considerations

No privacy concerns have been raised against this specification.

8. Security Considerations

Since the <image> value of shape-outside can expose some image data in a new way, use is limited to images with CORS approval.

Acknowledgments

This specification is made possible by input from Tab Atkins Jr., Amelia Bellamy-Royds, Oriol Brufau, Andrei Bucur, Alexandru Chiculita, Boris Chiou, Emilio Cobos Álvarez, Elika Etemad, Arron Eicholz, Sylvain Galineau, Daniel Glazman, Arno Gourdol, Zoltan Horvath, Chris Jones, Bem Jones-Bey, Ian Kilpatrick, Guillaume Lebas, Ting-Yu Lin, Eric Meyer, Marcus Mielke, Alex Mogilevsky, Hans Muller, Mihnea Ovidenie, Virgil Palanciuc, Noam Rosenthal, Robert Sanderson, Dirk Schulze, Jen Simmons, Peter Sorotokin, Bear Travis, Lea Verou, Eugene Veselov, Brad Werth, Stephen Zilles and the CSS Working Group members.

Change Log

Since 15 November 2022

Added Web Platform Test annotations
Changed fill-rule default for path() in SVG shapes
Clarified shape-margin and float area interaction
Added polygon rounding examples
Included all basic shapes in property prose for issue #9728
Added start of polygon vertex rounding
Used opacity-value (the value) not opacity (the property) as value of shape-image-threshold for issue #8311
Replaced <alpha-value> with <opacity>
Clarified that shape-outside applies to initial letter boxes for issue #5160
Rebased ellipse()/circle() definitions on top of from Images, for issue #824
Made it clear that the position defaulting can be overridden by other specs (like Motion).
Targeted fix of shape serialization section, fixed examples, for issue #8695
Fixed xywh() computed value for issue #9053
Rewrote syntax with implicitly optional comma PR #9650
Cleanup <*-box> definitions PR #9505
Added range notation in property definitions
Added vertex rounding for polygon() for issue #9843
Updated shape-outside prose to include all <basic-shape>s for issue #9728
Clarified the shape-margin contribution to float areas for issue #2949
Added fill-rule default handling for SVG shapes from issue #3468
Moved 'shape()' from level 2

Since March 20th 2014

Clarified shape-margin computed value
Clarified serialization of default position values, for issue #402
Clarified empty circles and ellipses for issue #850
Dropped the "Media:" entry from propdef tables, as with all CSS modules
Updated box model references from CSS 2 to CSS Box Model 3
Updated Computed Value and Animation Type in propdef tables
Clarified computed value of shape-outside property, for issue #4042
Clarified that shape-image-threshold can take a percentage value, like any alpha value, for issue #4102
Moved path() back from level 2
Added handling of negative margins for margin-box, for issue #675
Removed special-case serialization of position values, for issue #2301
Added clarifications to shape-margin examples
Added margin=0 case for margin-box shape, for issue #675
Changed rules about degenerate shapes to use shape edges, for issue #2375
Clarified that invalid path strings make the path() invalid, for issue #392
Rewrote definition of inset rectangles, added auto value, added examples
Clarified computed value of basic shape rect functions
Assorted markup fixes, including use of range notation and exporting defined terms for use in other specifications
Split Privacy and Security considerations into separate sections

Since February 11th 2014

Replaced divs with images in the first example
Add 0px to last serialization example

Since December 3rd 2013

Updated computed value and serialization of basic shapes
Added a margin-box example
Change auto to none for shape-outside
Defined shape-box instead of redefining box
Clarified that shape from image may produce more than one path

Since June 20th 2013

Added shape from box value section
Updated basic-shape interpolation
Allow negative insets, disallow negative radii
Changed relevant to reference
Remove box-sizing dependency, add relevant box keywords
Changed circle() and ellipse() to use radial gradient syntax
Postponed rectangle() to level 2
Clarified shape-from-image sizing and positioning
Change inset-rectangle() to inset()
Future-proof shape-image-threshold to possibly apply to luminance
Added CORS fetching to shape-outside URLs
Changed shape-outside value from <uri> to <image>
Remove 'percentages based on auto-sizing resolve to 0'
Change initial value of shape-image-threshold to 0.0
Change float positioning to be unaffected by shape-outside
Shapes on floats clipped to float’s margin box

Since May 3rd 2012

Postpone shapes from SVG elements to a future Shapes level
Postpone shape-inside to a future Shapes level
split exclusions from shapes into separate modules
added inset-rectangle() to basic shapes
Changed shape-inside overflow diagrams to show exclusion behavior
Changed shape-inside to contribute to the wrapping context
Defined exclusion edges relative to wrapping content’s writing mode
Made use of start, end, before and after consistent
Added interpolation for basic shapes
Changed basic shapes to depend on box specified with box-sizing
Added overflow behavior for shape-inside.
Added wrap-flow:minimum.
Clarified processing model.
Changed wrap-margin and wrap-padding to shape-margin and shape-padding.
Removed wrap shorthand.

Since December 13th 2011

Clarified processing model.
Clarified interaction with floats.
Clarified that an exclusion element establishes a new block formatting context.

Conformance

Document conventions

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:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

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.

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.

Partial implementations

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.

Implementations of Unstable and Proprietary Features

Non-experimental implementations

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.

CSS Shapes Module Level 1

Abstract

Status of this document

1. Introduction

1.1. Module Interactions

1.2. Values

1.3. Terminology

2. Relation to the box model and float behavior

3. Basic Shapes

3.1. Supported Shapes

3.1.1. The shape() Function

3.1.1.1. Using shape() to create responsive, parametric speech bubble

3.1.1.2. Interpolating the shape() Function

3.2. Computed Values of Basic Shapes

3.3. Serialization of Basic Shapes

3.4. Interpolation of Basic Shapes

4. Shapes from Image

5. Shapes from Box Values

6. Declaring Shapes

6.1. Float Area Shape: the shape-outside property

6.2. Choosing Image Pixels: the shape-image-threshold property

6.3. Expanding a Shape: the shape-margin property

7. Privacy Considerations

8. Security Considerations

Acknowledgments

Change Log

Since 15 November 2022

Since March 20th 2014

Since February 11th 2014

Since December 3rd 2013

Since June 20th 2013

Since May 3rd 2012

Since December 13th 2011

Conformance

Document conventions

Conformance classes

Partial implementations

Implementations of Unstable and Proprietary Features

Non-experimental implementations

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

Informative References

Property Index