Copyright © 2011 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc. This module contains the features of CSS level 3 relating to positioned type boxes with their margins, positioning properties, and the "out-of-flow" layout of boxes. Also included in the document are sections that are relevant to properly positioning elements using absolute positioning. It includes and extends the functionality of CSS level 2 [CSS21].
Other kinds of layout, such as tables, "floating" boxes, ruby annotations, grid layouts, columns and basic handling of normal "flow" content, are described in other modules. Also, the layout of text inside each line is defined elsewhere.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
The CSS WG maintains an issues list for this module. A test suite and implementation report for CSS Positioning Level 3 will be developed during the Candidate Recommendation phase, which will last a minimum of three months, i.e., at least until 15 December 2011. See the section "CR exit criteria" for more details.
This document was published by the CSS Working Group as an Editor's Draft. If you wish to make comments regarding this document, please send them to www-style@w3.org (subscribe, archives). All feedback is welcome.
Publication as a Editor's Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
Note, the model in this specification extends and clarifies the model described in the CSS level 2 revision 1 specification [CSS21].
CSS assumes the document layout is modeled as a tree of elements. Each element has an ordered list of zero or more child elements, with an optional string of text before the list, in-between the children and after the list. The unique element that has no parent is called the root element.
This module describes how any of the elements from the tree of elements can be arranged independent of document order (i.e. taken out of "flow").
With a positioned element the element may be placed anywhere within the content not specifically respecting the tree of elements order.
In [CSS21], the visual formatting model explained how each element in the document tree generates zero or more boxes according to the box model. This module further explains and extends the positioning scheme. The layout of these boxes is governed by:
The properties defined in this module apply to both continuous media and paged media.
This module replaced and extends the positioning scheme features defined in [CSS21] sections:
This specification follows the CSS property definition conventions from [CSS21].
All other values are defined in CSS Level 2 Revision 1 [CSS21].
All properties defined in this specification also accept the inherit keyword as their property value, but for readability it has not been listed explicitly.
User agents for continuous media generally offer users a viewport (a window or other viewing area on the screen) through which users consult a document. User agents may change the document's layout when the viewport is resized (see the initial containing block).
When the viewport is smaller than the area of the canvas on which the document is rendered, the user agent may offer a scrolling mechanism. There is at most one viewport per canvas, but user agents may render to more than one canvas (i.e., provide different views of the same document).
In CSS, many box positions and sizes are calculated with respect to the edges of a rectangular box called a containing block. In general, generated boxes act as containing blocks for descendant boxes; we say that a box "establishes" the containing block for its descendants. The phrase "a box's containing block" means "the containing block in which the box lives," not the one it generates.
Each box is given a position with respect to its containing block, but it is not confined by this containing block; it may overflow.
The position and size of an element's box(es) are sometimes calculated relative to a certain rectangle, called the containing block of the element. The containing block of an element is defined as follows:
‘position: absolute’, the containing block is established
by the nearest ancestor with a
'position' other than 'static', in the following way:
'ltr',
the top and left of the containing
block are the top and left content edges of the first box generated by
the ancestor,
and the bottom and right are the
bottom and right content edges of the last box of the ancestor.
'rtl',
the top and right are the top and
right edges of the first box generated by the ancestor,
and the bottom and left are the
bottom and left content edges of the last box of the ancestor.
Note, in some cases when a line wraps it may seem as if the left and right positions are swapped.
If there is no such ancestor, the containing block is the initial containing block.
In paged media, an absolutely positioned element is positioned relative to its containing block ignoring any page breaks (as if the document were continuous). The element may subsequently be broken over several pages.
For absolutely positioned content that resolves to a position on a page other than the page being laid out (the current page), or resolves to a position on the current page which has already been rendered for printing, printers may place the content:
Note, a block-level element that is split over several pages may have a different width on each page and that there may be device-specific limits.
With no positioning, the containing blocks (C.B.) in the following document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Illustration of containing blocks</TITLE>
</HEAD>
<BODY id="body">
<DIV id="div1">
<P id="p1">This is text in the first paragraph...</P>
<P id="p2">This is text <EM id="em1"> in the
<STRONG id="strong1">second</STRONG> paragraph.</EM></P>
</DIV>
</BODY>
</HTML>
are established as follows:
| For box generated by | C.B. is established by |
|---|---|
| html | initial C.B. (UA-dependent) |
| body | html |
| div1 | body |
| p1 | div1 |
| p2 | div1 |
| em1 | p2 |
| strong1 | p2 |
If we position "div1":
#div1 { position: absolute; left: 50px; top: 50px }
its containing block is no longer "body"; it becomes the initial containing block (since there are no other positioned ancestor boxes).
If we position "em1" as well:
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
the table of containing blocks becomes:
| For box generated by | C.B. is established by |
|---|---|
| html | initial C.B. (UA-dependent) |
| body | html |
| div1 | initial C.B. |
| p1 | div1 |
| p2 | div1 |
| em1 | div1 |
| strong1 | em1 |
By positioning "em1", its containing block becomes the nearest positioned ancestor box (i.e., that generated by "div1").
Boxes in the normal flow belong to a formatting context, which may be block or inline, but not both simultaneously. See the CSS Basic Box Model module [CSS3BOX] for further details about normal flow.
A float is a box that is positioned within content, often left or right on the current line. The most interesting characteristic of a float (or "floated" or "floating" box) is that content may flow along its side (or be prohibited from doing so by the 'clear' property) or around the floated box. See the CSS Floats and Exclusions spec for more details about Floats.
In CSS, a box may be laid out according to three positioning schemes:
An element is called out of flow if it is floated, absolutely positioned, or is the root element. An element is called in-flow if it is not out-of-flow. The flow of an element A is the set consisting of A and all in-flow elements whose nearest out-of-flow ancestor is A.
Once a box has been laid out according to the normal flow or floated, it may be shifted relative to this position. This is called relative positioning. Offsetting a box (B1) in this way has no effect on the box (B2) that follows: B2 is given a position as if B1 were not offset and B2 is not re-positioned after B1's offset is applied. This implies that relative positioning may cause boxes to overlap. However, if relative positioning causes an 'overflow: auto' or 'overflow: scroll' box to have overflow, the UA must allow the user to access this content (at its offset position), which, through the creation of scrollbars, may affect layout.
A relatively positioned box keeps its normal flow size, including line breaks and the space originally reserved for it.
A relatively positioned box establishes a new a new containing block for absolutely positioned descendants. (This is a common use of relatively positioned boxes.) The section on containing blocks explains when a relatively positioned box establishes a new containing block.
For relatively positioned elements, 'left' and 'right' move the box(es) horizontally, without changing their size. 'Left' moves the boxes to the right, and 'right' moves them to the left. Since boxes are not split or stretched as a result of 'left' or 'right', the used values are always: left = -right.
If both 'left' and 'right' are 'auto' (their initial values), the used values are '0' (i.e., the boxes stay in their original position).
If 'left' is 'auto', its used value is minus the value of 'right' (i.e., the boxes move to the left by the value of 'right').
If 'right' is specified as 'auto', its used value is minus the value of 'left'.
If neither 'left' nor 'right' is 'auto', the position is over-constrained, and one of them has to be ignored. If the 'direction' property of the containing block is 'ltr', the value of 'left' wins and 'right' becomes -'left'. If 'direction' of the containing block is 'rtl', 'right' wins and 'left' is ignored.
The following three rules are equivalent:
div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }
The 'top' and 'bottom' properties move relatively positioned element(s) up or down without changing their size. 'Top' moves the boxes down, and 'bottom' moves them up. Since boxes are not split or stretched as a result of 'top' or 'bottom', the used values are always: top = -bottom.
If 'top' and 'bottom' are 'auto', their used values are both '0'.
If one of them, 'top' or 'bottom', is 'auto', the 'auto' value becomes the negative of the other.
If neither 'top' and 'bottom' is 'auto', 'bottom' is ignored (i.e., the used value of 'bottom' will be minus the value of 'top').
Note, dynamic movement of relatively positioned boxes can produce animation effects in scripting environments (see also the 'visibility' property). Although relative positioning may be used as a form of superscripting and subscripting, the line height is not automatically adjusted to take the positioning into consideration. See the description of line height calculations for more information.
Examples of relative positioning are provided in the section comparing normal flow, floats, and absolute positioning.
In the absolute positioning model, a box is explicitly offset with respect to its containing block. It is removed from the normal flow entirely (it has no impact on later siblings). An absolutely positioned box establishes a new containing block for normal flow children and absolutely (but not fixed) positioned descendants. However, the contents of an absolutely positioned element do not flow around any other boxes. They may obscure the contents of another box (or be obscured themselves), depending on the stack levels of the overlapping boxes.
References in this specification to an absolutely positioned element (or its box) imply that the element's 'position' property has the value 'absolute' or 'fixed'.
In the page positioning model, a box is explicitly offset with respect to its initial containing block. It is removed from the normal flow entirely (it has no impact on later siblings). A page positioned box establishes a new containing block for normal flow children and absolutely (but not fixed, or page) positioned descendants. However, the contents of a page positioned element do not flow around any other boxes. They may obscure the contents of another box (or be obscured themselves), depending on the stack levels of the overlapping boxes. For paged media, boxes with page positions are only generated on the initial page where the page position element exists. Boxes with page position that are larger than the page area are clipped and the remaining part of the box is placed on the following page.
References in this specification to a page positioned element (or its box) imply that the element's 'position' property has the value 'page'.
Fixed positioning is a subcategory of absolute positioning. The only difference is that for a fixed positioned box, the containing block is established by the viewport. For continuous media, fixed boxes do not move when the document is scrolled. In this respect, they are similar to fixed background images. For paged media, boxes with fixed positions are repeated on every page. This is useful for placing, for instance, a signature at the bottom of each page. Boxes with fixed position that are larger than the page area are clipped. Parts of the fixed position box that are not visible in the initial containing block will not print.
Authors may use fixed positioning to create frame-like presentations. Consider the following frame layout:
This might be achieved with the following HTML document and style rules:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>A frame document with CSS 2.1</TITLE>
<STYLE type="text/css" media="screen">
BODY { height: 8.5in } /* Required for percentage heights below */
#header {
position: fixed;
width: 100%;
height: 15%;
top: 0;
right: 0;
bottom: auto;
left: 0;
}
#sidebar {
position: fixed;
width: 10em;
height: auto;
top: 15%;
right: auto;
bottom: 100px;
left: 0;
}
#main {
position: fixed;
width: auto;
height: auto;
top: 15%;
right: 0;
bottom: 100px;
left: 10em;
}
#footer {
position: fixed;
width: 100%;
height: 100px;
top: auto;
right: 0;
bottom: 0;
left: 0;
}
</STYLE>
</HEAD>
<BODY>
<DIV id="header"> ... </DIV>
<DIV id="sidebar"> ... </DIV>
<DIV id="main"> ... </DIV>
<DIV id="footer"> ... </DIV>
</BODY>
</HTML>
The 'position' and 'float' properties determine which of the CSS 2.1 positioning algorithms is used to calculate the position of a box.
| Value: | static | relative | absolute | page | fixed |
| Initial: | static |
| Applies to: | all elements except table-column-group and table-column |
| Inherited: | no |
| Percentages: | N/A |
| Media: | visual |
| Computed value: | as specified |
The values of this property have the following meanings:
@media screen {
h1#first { position: fixed }
}
@media print {
h1#first { position: static }
}
UAs must not paginate the content of fixed boxes. Note that UAs may print invisible content in other ways. See "Content outside the page box" in chapter 13.
In previous versions of CSS user agents may treat position ‘fixed’ as 'static' on the root element. CSS3 removes that option and it is now required that the root element is ‘fixed’ as defined.
An element is said to be positioned if its 'position' property has a value other than 'static'. Positioned elements generate positioned boxes, laid out according to four properties:
| Value: | <length> | <percentage> | auto |
| Initial: | auto |
| Applies to: | positioned elements |
| Inherited: | no |
| Percentages: | refer to height of containing block |
| Media: | visual |
| Computed value: |
for ‘position: relative’, see Relative Positioning.
For ‘position: static’, ‘auto’.
Otherwise: if specified as a length, the corresponding
absolute length; if specified as a percentage, the specified value;
otherwise, ‘auto’.
|
This property specifies how far an absolutely positioned box's top margin edge is offset below the top edge of the box's containing block. For relatively positioned boxes, the offset is with respect to the top edges of the box itself (i.e., the box is given a position in the normal flow, then offset from that position according to these properties). For page positioned boxes, the offset is with respect to the top edge of the initial containing box itself. For absolutely positioned elements whose containing block is based on a block-level element, these properties are an offset from the padding edge of that element.
| Value: | <length> | <percentage> | auto |
| Initial: | auto |
| Applies to: | positioned elements |
| Inherited: | no |
| Percentages: | refer to width of containing block |
| Media: | visual |
| Computed value: |
for ‘position: relative’, see Relative Positioning.
For ‘position: static’, ‘auto’.
Otherwise: if specified as a length, the corresponding
absolute length; if specified as a percentage, the specified value;
otherwise, ‘auto’.
|
Like 'top', but specifies how far a box's right margin edge is offset to the left of the right edge of the box's containing block. For relatively positioned boxes, the offset is with respect to the right edge of the box itself. For page positioned boxes, the offset is with respect to the right edge of the initial containing box itself. For absolutely positioned elements whose containing block is based on a block-level element, these properties are an offset from the padding edge of that element.
| Value: | <length> | <percentage> | auto |
| Initial: | auto |
| Applies to: | positioned elements |
| Inherited: | no |
| Percentages: | refer to height of containing block |
| Media: | visual |
| Computed value: |
for ‘position: relative’, see Relative Positioning.
For ‘position: static’, ‘auto’.
Otherwise: if specified as a length, the corresponding
absolute length; if specified as a percentage, the specified value;
otherwise, ‘auto’.
|
Like 'top', but specifies how far a box's bottom margin edge is offset above the bottom of the box's containing block. For relatively positioned boxes, the offset is with respect to the bottom edge of the box itself. For page positioned boxes, the offset is with respect to the bottom edge of the initial containing box itself. For absolutely positioned elements whose containing block is based on a block-level element, these properties are an offset from the padding edge of that element.
| Value: | <length> | <percentage> | auto |
| Initial: | auto |
| Applies to: | positioned elements |
| Inherited: | no |
| Percentages: | refer to width of containing block |
| Media: | visual |
| Computed value: |
for ‘position: relative’, see Relative Positioning.
For ‘position: static’, ‘auto’.
Otherwise: if specified as a length, the corresponding
absolute length; if specified as a percentage, the specified value;
otherwise, ‘auto’.
|
Like 'top', but specifies how far a box's left margin edge is offset to the right of the left edge of the box's containing block. For relatively positioned boxes, the offset is with respect to the left edge of the box itself.
The values for the four properties have the following meanings:
Note for 'page' and 'fixed' positioned elements using large values or negative values may easily move elements outside the viewport and make the contents unreachable through scrolling or other means. Authors should be aware that page and fixed position elements are always relative to the initial containing block.
The constraint that determines the used values for these elements is:
'left' + 'margin-left' + 'border-left-width' +
'padding-left' + 'width' + 'padding-right' + 'border-right-width' +
'margin-right' + 'right' = width of containing block
If all three of 'left', 'width', and 'right' are 'auto': First set any 'auto' values for 'margin-left' and 'margin-right' to 0. Then, if the 'direction' property of the element establishing the static-position containing block is 'ltr' set 'left' to the static position and apply rule number three below; otherwise, set 'right' to the static-position and apply rule number one below.
If none of the three is 'auto': If both 'margin-left' and 'margin-right' are 'auto', solve the equation under the extra constraint that the two margins get equal values, unless this would make them negative, in which case when direction of the containing block is 'ltr' ('rtl'), set 'margin-left' ('margin-right') to zero and solve for 'margin-right' ('margin-left'). If one of 'margin-left' or 'margin-right' is 'auto', solve the equation for that value. If the values are over-constrained, ignore the value for 'left' (in case the 'direction' property of the containing block is 'rtl') or 'right' (in case 'direction' is 'ltr') and solve for that value.
Otherwise, set 'auto' values for 'margin-left' and 'margin-right' to 0, and pick one of the following six rules that apply.
If 'height' and 'width' both have computed values of 'auto' and the element also has an intrinsic width, then that intrinsic width is the used value of 'width'.
If 'height' and 'width' both have computed values of 'auto' and the element has no intrinsic width, but does have an intrinsic height and intrinsic ratio; or if 'width' has a computed value of 'auto', 'height' has some other computed value, and the element does have an intrinsic ratio; then the used value of 'width' is:
(used height) * (intrinsic ratio)
If 'height' and 'width' both have computed values of 'auto', the element has an intrinsic ratio but no intrinsic height or width, and the containing block's width does not itself depend on the replaced element's width, then the used value of 'width' is calculated from the constraint equation used for block-level, non-replaced elements in normal flow.
Otherwise, if 'width' has a computed value of 'auto', and the element has an intrinsic width, then that intrinsic width is the used value of 'width'.
Otherwise, if 'width' has a computed value of 'auto', but none of the conditions above are met, and then the used value of 'width' becomes 300px. If 300px is too wide to fit the device, UAs should use the width of the largest rectangle that has a 2:1 ratio and fits the device instead.
After establishing the 'width', in order to position the replaced element, apply the following rules as appropriate.
For absolutely positioned elements, the used values of the vertical dimensions must satisfy this constraint:
'top' + 'margin-top' + 'border-top-width' +
'padding-top' + 'height' + 'padding-bottom' + 'border-bottom-width' +
'margin-bottom' + 'bottom' = height of containing block
If all three of 'top', 'height', and 'bottom' are auto, set 'top' to the static position and apply rule number three below.
If none of the three are 'auto': If both 'margin-top' and 'margin-bottom' are 'auto', solve the equation under the extra constraint that the two margins get equal values. If one of 'margin-top' or 'margin-bottom' is 'auto', solve the equation for that value. If the values are over-constrained, ignore the value for 'bottom' and solve for that value.
Otherwise, pick the one of the following six rules that applies.
If 'height' and 'width' both have computed values of 'auto' and the element also has an intrinsic height, then that intrinsic height is the used value of 'height'.
Otherwise, if 'height' has a computed value of 'auto' and the element has an intrinsic ratio then the used value of 'height' is:
(used width) / (intrinsic ratio)
Otherwise, if 'height' has a computed value of 'auto' and the element has an intrinsic height, then that intrinsic height is the used value of 'height'.
Otherwise, if 'height' has a computed value of 'auto', but none of the conditions above are met, then the used value of 'height' must be set to the height of the largest rectangle that has a 2:1 ratio, has a height not greater than 150px, and has a width not greater than the device width.
After establishing the 'height', in order to position the replaced element, apply the following rules as appropriate.
In certain cases (see, e.g., The height of absolute, page or fixed positioned, non-replaced element above), the height of an element that establishes a block formatting context is computed as follows:
If it only has inline-level children, the height is the distance between the top of the topmost line box and the bottom of the bottommost line box.
If it has block-level children, the height is the distance between the top margin-edge of the topmost block-level child box and the bottom margin-edge of the bottommost block-level child box.
Absolutely positioned children are ignored, and relatively positioned boxes are considered without their offset. Note that the child box may be an anonymous block box.
In addition, if the element has any floating descendants whose bottom margin edge is below the element's bottom content edge, then the height is increased to include those edges. Only non-positioned floats that participate in this block formatting context are taken into account, e.g., floats inside absolutely positioned descendants or other floats are not.
The three properties that affect box generation and layout — 'display', 'position', and 'float' — interact as follows:
| Specified value | Computed value |
|---|---|
| inline-table | table |
| inline, table-row-group, table-column, table-column-group, table-header-group, table-footer-group, table-row, table-cell, table-caption, inline-block | block |
| others | same as specified |
This section is non-normative.
To illustrate the differences between normal flow, relative positioning, floats, and absolute positioning, we provide a series of examples based on the following HTML:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Comparison of positioning schemes</TITLE>
</HEAD>
<BODY>
<P>Beginning of body contents.
<SPAN id="outer"> Start of outer contents.
<SPAN id="inner"> Inner contents.</SPAN>
End of outer contents.</SPAN>
End of body contents.
</P>
</BODY>
</HTML>
In this document, we assume the following rules:
body { display: block; font-size:12px; line-height: 200%;
width: 400px; height: 400px }
p { display: block }
span { display: inline }
The final positions of boxes generated by the outer and inner elements vary in each example. In each illustration, the numbers to the left of the illustration indicate the normal flow position of the double-spaced (for clarity) lines.
Note. The diagrams in this section are illustrative and not to scale. They are meant to highlight the differences between the various positioning schemes in CSS 2.1, and are not intended to be reference renderings of the examples given.
Consider the following CSS declarations for outer and inner that do not alter the normal flow of boxes:
#outer { color: red }
#inner { color: blue }
The P element contains all inline content: anonymous inline text and two SPAN elements. Therefore, all of the content will be laid out in an inline formatting context, within a containing block established by the P element, producing something like:
To see the effect of relative positioning, we specify:
#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }
Text flows normally up to the outer element. The outer text is then flowed into its normal flow position and dimensions at the end of line 1. Then, the inline boxes containing the text (distributed over three lines) are shifted as a unit by '-12px' (upwards).
The contents of inner, as a child of outer, would normally flow immediately after the words "of outer contents" (on line 1.5). However, the inner contents are themselves offset relative to the outer contents by '12px' (downwards), back to their original position on line 2.
Note that the content following outer is not affected by the relative positioning of outer.
Note also that had the offset of outer been '-24px', the text of outer and the body text would have overlapped.
Now consider the effect of floating the inner element's text to the right by means of the following rules:
#outer { color: red }
#inner { float: right; width: 130px; color: blue }
Text flows normally up to the inner box, which is pulled out of the flow and floated to the right margin (its 'width' has been assigned explicitly). Line boxes to the left of the float are shortened, and the document's remaining text flows into them.
To show the effect of the 'clear' property, we add a sibling element to the example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Comparison of positioning schemes II</TITLE>
</HEAD>
<BODY>
<P>Beginning of body contents.
<SPAN id=outer> Start of outer contents.
<SPAN id=inner> Inner contents.</SPAN>
<SPAN id=sibling> Sibling contents.</SPAN>
End of outer contents.</SPAN>
End of body contents.
</P>
</BODY>
</HTML>
The following rules:
#inner { float: right; width: 130px; color: blue }
#sibling { color: red }
cause the inner box to float to the right as before and the document's remaining text to flow into the vacated space:
However, if the 'clear' property on the sibling element is set to 'right' (i.e., the generated sibling box will not accept a position next to floating boxes to its right), the sibling content begins to flow below the float:
#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }
Next, we consider the effect of absolute positioning. Consider the following CSS declarations for outer and inner:
#outer {
position: absolute;
top: 200px; left: 200px;
width: 200px;
color: red;
}
#inner { color: blue }
which cause the top of the outer box to be positioned with respect to its containing block. The containing block for a positioned box is established by the nearest positioned ancestor (or, if none exists, the initial containing block, as in our example). The top side of the outer box is '200px' below the top of the containing block and the left side is '200px' from the left side. The child box of outer is flowed normally with respect to its parent.
The following example shows an absolutely positioned box that is a child of a relatively positioned box. Although the parent outer box is not actually offset, setting its 'position' property to 'relative' means that its box may serve as the containing block for positioned descendants. Since the outer box is an inline box that is split across several lines, the first inline box's top and left edges (depicted by thick dashed lines in the illustration below) serve as references for 'top' and 'left' offsets.
#outer {
position: relative;
color: red
}
#inner {
position: absolute;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
This results in something like the following:
If we do not position the outer box:
#outer { color: red }
#inner {
position: absolute;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
the containing block for inner becomes the initial containing block (in our example). The following illustration shows where the inner box would end up in this case.
Relative and absolute positioning may be used to implement change bars, as shown in the following example. The following fragment:
<P style="position: relative; margin-right: 10px; left: 10px;">
I used two red hyphens to serve as a change bar. They
will "float" to the left of the line containing THIS
<SPAN style="position: absolute; top: auto; left: -1em; color: red;">--</SPAN>
word.</P>
might result in something like:
First, the paragraph (whose containing block sides are shown in the illustration) is flowed normally. Then it is offset '10px' from the left edge of the containing block (thus, a right margin of '10px' has been reserved in anticipation of the offset). The two hyphens acting as change bars are taken out of the flow and positioned at the current line (due to 'top: auto'), '-1em' from the left edge of its containing block (established by the P in its final position). The result is that the change bars seem to "float" to the left of the current line.
Finally, we consider the effect of page positioning. Consider the following CSS declarations for outer and inner:
#outer {
position: page;
top: 200px; left: 200px;
width: 200px;
color: red;
}
#inner { color: blue }
which cause the top of the outer box to be positioned with respect to its containing block. The containing block for a page positioned box is always established by the initial containing block. The top side of the outer box is '200px' below the top of the initial containing block and the left side is '200px' from the left side. The child box of outer is flowed normally with respect to its parent.
The following example shows a page positioned box that is a child of a relatively positioned box. Although the parent outer box is setting its 'position' property to 'relative' this outer box box does not serve as the containing block for page positioned descendants. Since the inner box is a page positioned element its containing block is not the relative positioned outer box box, page positioned elements are positioned from the initial containing block, in this case the top and left edges (depicted by thick dashed lines in the illustration below) serve as references for 'top' and 'left' offsets.
#outer {
position: relative;
color: red
}
#inner {
position: page;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
This results in something like the following:
The previous image is incorrect. Need to create a new image.
If we do not position the outer box:
#outer { color: red }
#inner {
position: page;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
the containing block for inner becomes the initial containing block. The following illustration shows where the inner box would end up in this case.
A clipping region defines what portion of an element's border box is visible. By default, the element is not clipped. However, the clipping region may be explicitly set with the 'clip' property.
| Value: | <shape> | auto | |
| Initial: | auto |
| Applies to: | absolutely positioned elements |
| Inherited: | no |
| Percentages: | N/A |
| Media: | visual |
| Computed value: | 'auto' if specified as 'auto', otherwise a rectangle with four values, each of which is 'auto' if specified as 'auto' and the computed length otherwise |
The 'clip' property applies only to absolutely positioned elements. Values have the following meanings:
Each of the arguments <top> and <bottom> specify offsets from the top border edge of the box, and <right>, and <left> specify offsets from the left border edge of the box. Authors should separate offset values with commas. User agents must support separation with commas, but may also support separation without commas (but not a combination), because a previous revision of this specification was ambiguous in this respect.
<top>, <right>,<bottom>, and <left> may either have a <length> value or 'auto'. Negative lengths are permitted. The value 'auto' means that a given edge of the clipping region will be the same as the edge of the element's generated border box (i.e., 'auto' means the same as '0' for <top> and <left>, the same as the used value of the height plus the sum of vertical padding and border widths for <bottom>, and the same as the used value of the width plus the sum of the horizontal padding and border widths for <right>, such that four 'auto' values result in the clipping region being the same as the element's border box).
When coordinates are rounded to pixel coordinates, care should be taken that no pixels remain visible when <left> and <right> have the same value (or <top> and <bottom> have the same value), and conversely that no pixels within the element's border box remain hidden when these values are 'auto'.
Like rect(), except that the values are offsets relative to the respective edges of the element.
Diagram of the rectangles defined by rect() and inset-rect().
An element's clipping region clips out any aspect of the element (e.g., content, children, background, borders, text decoration, outline and visible scrolling mechanism — if any) that is outside the clipping region. Content that has been clipped does not cause overflow.
The element's ancestors may also clip portions of their content (e.g., via their own 'clip' property and/or if their 'overflow' property is not 'visible'); what is rendered is the cumulative intersection.
If the clipping region exceeds the bounds of the UA's document window, content may be clipped to that window by the native operating environment.
Example: The following two rules:
p#one { clip: rect(5px, 40px, 45px, 5px); }
p#two { clip: rect(5px, 55px, 45px, 5px); }and assuming both Ps are 50 by 55 px, will create, respectively, the rectangular clipping regions delimited by the dashed lines in the following illustrations:
The following rule:
IMG { clip: rect(5px, 40px, 45px, 5px); }
will create the rectangular clipping region delimited by the dashed line in the following illustration:
An image of 50 by 55 pixels is clipped with a clipping mask of 35 by 40 pixels.
Note. In CSS 2.1, all clipping regions are rectangular. We anticipate future extensions to permit non-rectangular clipping.
In the following sections, the expression "in front of" means closer to the user as the user faces the screen.
In CSS, each box has a position in three dimensions. In addition to their horizontal and vertical positions, boxes lie along a "z-axis" and are formatted one on top of the other. Z-axis positions are particularly relevant when boxes overlap visually. This section discusses how boxes may be positioned along the z-axis.
Each box belongs to one stacking context . Each box in a given stacking context has an integer stack level , which is its position on the z-axis relative to other boxes in the same stacking context. Boxes with greater stack levels are always formatted in front of boxes with lower stack levels. Boxes may have negative stack levels. Boxes with the same stack level in a stacking context are stacked bottom-to-top according to document tree order.
The rootelement creates a root stacking context , but other elements may establish local stacking contexts . Stacking contexts are inherited. A local stacking context is atomic; boxes in other stacking contexts may not come between any of its boxes.
An element that establishes a local stacking context generates a box
that has two stack levels: one for the stacking context it creates
(always ‘0’) and one for the stacking
context to which it belongs (given by the ‘z-index’ property).
An element's box has the same stack level as its parent's box unless given a different stack level with the ‘z-index’ property.
For a positioned box, the 'z-index' property specifies:
Values have the following meanings:
In the following example, the stack levels of the boxes (named with their "id" attributes) are: "text2"=0, "image"=1, "text3"=2, and "text1"=3. The "text2" stack level is inherited from the root box. The others are specified with the 'z-index' property.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Z-order positioning</TITLE>
<STYLE type="text/css">
.pile {
position: absolute;
left: 2in;
top: 2in;
width: 3in;
height: 3in;
}
</STYLE>
</HEAD>
<BODY>
<P>
<IMG id="image" class="pile"
src="butterfly.png" alt="A butterfly image"
style="z-index: 1">
<DIV id="text1" class="pile"
style="z-index: 3">
This text will overlay the butterfly image.
</DIV>
<DIV id="text2">
This text will be beneath everything.
</DIV>
<DIV id="text3" class="pile"
style="z-index: 2">
This text will underlay text1, but overlay the butterfly image
</DIV>
</BODY>
</HTML>
This example demonstrates the notion of transparency. The default behavior of the background is to allow boxes behind it to be visible. In the example, each box transparently overlays the boxes below it. This behavior can be overridden by using one of the existing background properties.
The bottom of the stack is the furthest from the user, the top of the stack is the nearest to the user:
Schematic diagram of a stacking context with four layers.
The stacking context background and most negative positioned stacking contexts are at the bottom of the stack, while the most positive positioned stacking contexts are at the top of the stack.
The canvas is transparent if contained within another, and given a UA-defined color if it is not. It is infinite in extent and contains the root element. Initially, the viewport is anchored with its top left corner at the canvas origin.
The painting order order for the descendants of an element generating a stacking context (see the 'z-index' property) is:
Note, some of the boxes may have been generated by line splitting or the Unicode bidirectional algorithm.
Need to account for new stacking context creation by opacity and transforms.
The background of the root element is only painted once, over the whole canvas.
While the backgrounds of bidirectional in lines are painted in tree order, they are positioned in visual order. Since the positioning of inline backgrounds is unspecified in CSS, the exact result of these two requirements is UA-defined. A future version of CSS may define this in more detail.
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 CSS Backgrounds and Borders Level 3 is defined for three classes:
A style sheet is conformant to CSS Backgrounds and Borders Level 3 if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.
A renderer is conformant to CSS Backgrounds and Borders Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the properties defined by CSS Backgrounds and Borders Level 3 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 CSS Backgrounds and Borders Level 3 if it writes syntactically correct style sheets, according to the generic CSS grammar and the individual grammars of each property in this module.
This section is non-normative.
CSS has different levels of features, each a subset of the other. (See [CSSBEIJING] for a full explanation.) The lists below describe which features from this specification are in each level.
All features described in the CSS Positioning Module Level 3
As described in the W3C process document, a Candidate Recommendation (CR) is a specification that W3C recommends for use on the Web. The next stage is “Recommendation,” when the specification is sufficiently implemented.
For this specification to be proposed as a W3C Recommendation, the following conditions shall be met. There must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:
A minimum of three months of the CR period must have elapsed. This is to ensure that enough time is given for any remaining major errors to be caught.
Features will be dropped if two or more interoperable implementations are not found by the end of the CR period.
Features may/will also be dropped if adequate/sufficient (by judgment of CSS WG) tests have not been produced for those feature(s) by the end of the CR period.
This spec would not have been possible without input and support from many helpful people. Special thanks to Bert Bos, Rossen Atanassov, Chris Jones, John Jansen. Thanks to Eliot Graff for editorial assistance.
No informative references.