Contents
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:
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.
Note that 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 4.0 Transitional//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").
| Value: | <length> | <percentage> | auto | inherit |
| Initial: | auto |
| Applies to: | all elements but non-replaced inline elements, table rows, and row groups |
| Inherited: | no |
| Percentages: | refer to width of containing block |
| Media: | visual |
| Computed value: | the percentage as specified or the absolute length; 'auto' if the property does not apply |
This property specifies the content width of boxes generated by block-level and replaced elements.
This property does not apply to non-replaced inline-level elements. The content width of a non-replaced inline element's boxes is that of the rendered content within them (before any relative offset of children). Recall that inline boxes flow into line boxes. The width of line boxes is given by the their containing block, but may be shorted by the presence of floats.
The width of a replaced element's box is intrinsic and may be scaled by the user agent if the value of this property is different than 'auto'.
Values have the following meanings:
Negative values for 'width' are illegal.
For example, the following rule fixes the content width of paragraphs at 100 pixels:
p { width: 100px }
The values of an element's 'width', 'margin-left', 'margin-right', 'left' and 'right' properties as used for layout depend on the type of box generated and on each other. (The value used for layout is sometimes referred to as the used value.) In principle, the values used are the same as the computed values, with 'auto' replaced by some suitable value, and percentages calculated based on the containing block, but there are exceptions. The following situations need to be distinguished:
For Points 1-6 and 9-10, the values of 'left' and 'right' used for layout are determined by the rules in section 9.4.3.
The 'width' property does not apply. A computed value of 'auto' for 'left', 'right', 'margin-left' or 'margin-right' becomes a used value of '0'.
A computed value of 'auto' for 'margin-left' or 'margin-right' becomes a used value of '0'. If 'width' has a computed value of 'auto' and 'height' also has a computed value of 'auto', the element's intrinsic width is the used value of 'width'. If 'width' has a computed value of 'auto' and 'height' has some other computed value, then the used value of 'width' is:
(intrinsic width) * ( (used height) / (intrinsic height) )
The following constraints must hold between the used values of the other properties:
'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' = width of containing block
If all of the above have a computed value other than 'auto', the values are said to be "over-constrained" and one of the used values will have to be different from its computed value. If the 'direction' property has the value 'ltr', the specified value of 'margin-right' is ignored and the value is calculated so as to make the equality true. If the value of 'direction' is 'rtl', this happens to 'margin-left' instead.
If there is exactly one value specified as 'auto', its used value follows from the equality.
If 'width' is set to 'auto', any other 'auto' values become '0' and 'width' follows from the resulting equality.
If both 'margin-left' and 'margin-right' are 'auto', their used values are equal. This horizontally centers the element with respect to the edges of the containing block.
The used value of 'width' is determined as for inline replaced elements. If one of the margins is 'auto', its used value is given by the constraints above. Furthermore, if both margins are 'auto', their used values are equal.
If 'margin-left', or 'margin-right' are computed as 'auto', their used value is '0'.
If 'width' is computed as 'auto', the used value is the "shrink-to-fit" width.
Calculation of the shrink-to-fit width is similar to calculating the width of a table cell using the automatic table layout algorithm. Roughly: calculate the preferred width by formatting the content without breaking lines other than where explicit line breaks occur, and also calculate the preferred minimum width, e.g., by trying all possible line breaks. CSS 2.1 does not define the exact algorithm. Thirdly, find the available width: in this case, this is the width of the containing block minus 'margin-left' and 'margin-right'.
Then the shrink-to-fit width is: min(max(preferred minimum width, available width), preferred width).
If 'margin-left' or 'margin-right' are computed as 'auto', their used value is '0'. The used value of 'width' is determined as for inline replaced elements.
For the purposes of this section and the next, the term "static position" (of an element) refers, roughly, to the position an element would have had in the normal flow. More precisely:
But rather than actually calculating the dimensions of that hypothetical box, user agents are free to make a guess at its probable position.
For the purposes of calculating the static position, the containing block of fixed positioned elements is the initial containing block instead of the viewport.
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 'direction' 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. 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 'direction' 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 the one of the following six rules that applies.
Calculation of the shrink-to-fit width is similar to calculating the width of a table cell using the automatic table layout algorithm. Roughly: calculate the preferred width by formatting the content without breaking lines other than where explicit line breaks occur, and also calculate the preferred minimum width, e.g., by trying all possible line breaks. CSS 2.1 does not define the exact algorithm. Thirdly, calculate the available width: this is found by solving for 'width' after setting 'left' (in case 1) or 'right' (in case 3) to 0.
Then the shrink-to-fit width is: min(max(preferred minimum width, available width), preferred width).
This situation is similar to the previous one, except that the element has an intrinsic width. The sequence of substitutions is now:
If 'width' is 'auto', the used value is the shrink-to-fit width as for floating elements.
A computed value of 'auto' for 'margin-left' or 'margin-right' becomes a used value of '0'.
Exactly as inline replaced elements.
| Value: | <length> | <percentage> | inherit |
| Initial: | 0 |
| Applies to: | all elements except non-replaced inline elements and table elements |
| Inherited: | no |
| Percentages: | refer to width of containing block |
| Media: | visual |
| Computed value: | the percentage as specified or the absolute length |
| Value: | <length> | <percentage> | none | inherit |
| Initial: | none |
| Applies to: | all elements except non-replaced inline elements and table elements |
| Inherited: | no |
| Percentages: | refer to width of containing block |
| Media: | visual |
| Computed value: | the percentage as specified or the absolute length or 'none' |
These two properties allow authors to constrain box widths to a certain range. Values have the following meanings:
Negative values for 'min-width' and 'max-width' are illegal.
The following algorithm describes how the two properties influence the used value of the 'width' property:
However, for replaced elements with both 'width' and 'height' specified as 'auto', the algorithm is as follows:
| Value: | <length> | <percentage> | auto | inherit |
| Initial: | auto |
| Applies to: | all elements but non-replaced inline elements, table columns, and column groups |
| Inherited: | no |
| Percentages: | see prose |
| Media: | visual |
| Computed value: | the percentage as specified or the absolute length; 'auto' if the property does not apply |
This property specifies the content height of boxes generated by block-level, inline-block and replaced elements.
This property does not apply to non-replaced inline-level elements. See the section on computing heights and margins for non-replaced inline elements for the rules used instead.
Values have the following meanings:
Note that the height of the containing block of an absolutely positioned element is independent of the size of the element itself, and thus a percentage height on such an element can always be resolved. However, it may be that the height is not known until elements that come later in the document have been processed.
Negative values for 'height' are illegal.
For example, the following rule sets the content height of paragraphs to 100 pixels:
p { height: 100px }
Paragraphs of which the height of the contents exceeds 100 pixels will overflow according to the 'overflow' property.
For calculating the values of 'top', 'margin-top', 'height', 'margin-bottom', and 'bottom' a distinction must be made between various kinds of boxes:
For Points 1-6 and 9-10, the used values of 'top' and 'bottom' are determined by the rules in section 9.4.3.
The 'height' property doesn't apply. The height of the content area should be based on the font, but this specification does not specify how. A UA may, e.g., use the em-box or the maximum ascender and descender of the font. (The latter would ensure that glyphs with parts above or below the em-box still fall within the content area, but leads to differently sized boxes for different fonts; the formed would ensure authors can control background styling relative to the 'line-height', but leads to glyphs painting outside their content area.)
Note: level 3 of CSS will probably include a property to select which measure of the font is used for the content height.
The vertical padding, border and margin of an inline, non-replaced box start at the top and bottom of the content area, not the 'line-height'. But only the 'line-height' is used to calculate the height of the line box.
If more than one font is used (this could happen when glyphs are found in different fonts), the height of the content area is not defined by this specification. However, we suggest that the height is chosen such that the content area is just high enough for either (1) the em-boxes, or (2) the maximum ascenders and descenders, of all the fonts in the element. Note that this may be larger than any of the font sizes involved, depending on the baseline alignment of the fonts.
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0. If 'height' has a computed value of 'auto' and 'width' also has a computed value of 'auto', the element's intrinsic height is the used value of 'height'. If 'height' has a computed value of 'auto' and 'width' has some other computed value, then the used value of 'height' is:
(intrinsic height) * ( (used width) / (intrinsic width) )
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0. If 'height' is 'auto', the height depends on whether the element has any block-level children and whether it has padding or borders:
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 border-edge of the topmost block-level child box that doesn't have margins collapsed through it and the bottom border-edge of the bottommost block-level child box that doesn't have margins collapsed through it. However, if the element has a non-zero top padding and/or top border, then the content starts at the top margin edge of the topmost child. (The first case expresses the fact that the top and bottom margins of the element collapse with those of the topmost and bottommost children, while in the second case the presence of the padding/border prevents the top margins from collapsing.) Similarly, if the element has a non-zero bottom padding and/or bottom border, then the content ends at the bottom margin edge of the bottommost child.Only children in the normal flow are taken into account (i.e., floating boxes and absolutely positioned boxes are ignored, and relatively positioned boxes are considered without their offset). Note that the child box may be an anonymous block box.
For the purposes of this section and the next, the term "static position" (of an element) refers, roughly, to the position an element would have had in the normal flow. More precisely, the static position for 'top' is the distance from the top edge of the containing block to the top margin edge of a hypothetical box that would have been the first box of the element if its 'position' property had been 'static'. The value is negative if the hypothetical box is above the containing block.
But rather than actually calculating the dimensions of that hypothetical box, user agents are free to make a guess at its probable position.
For the purposes of calculating the static position, the containing block of fixed positioned elements is the initial containing block instead of the viewport.
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.
This situation is similar to the previous one, except that the element has an intrinsic height. The sequence of substitutions is now:
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0. If 'height' is 'auto', the height depends on the element's descendants:
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. (Note that the margins of the float and its children do not collapse together.)
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 top margin edge is above the top established above or whose bottom margin edge is below the bottom, then the height is increased to include those edges. Only floats that are children of the element itself or of descendants in the normal flow are taken into account, i.e., floats inside absolutely positioned descendants are not.
It is sometimes useful to constrain the height of elements to a certain range. Two properties offer this functionality:
| Value: | <length> | <percentage> | inherit |
| Initial: | 0 |
| Applies to: | all elements except non-replaced inline elements and table elements |
| Inherited: | no |
| Percentages: | see prose |
| Media: | visual |
| Computed value: | the percentage as specified or the absolute length |
| Value: | <length> | <percentage> | none | inherit |
| Initial: | none |
| Applies to: | all elements except non-replaced inline elements and table elements |
| Inherited: | no |
| Percentages: | see prose |
| Media: | visual |
| Computed value: | the percentage as specified or the absolute length or 'none' |
These two properties allow authors to constrain box heights to a certain range. Values have the following meanings:
Negative values for 'min-height' and 'max-height' are illegal.
The following algorithm describes how the two properties influence the computed value of the 'height' property:
However, for replaced elements with both 'width' and 'height' computed as 'auto', use the algorithm under Minimum and maximum widths above to find the used width and height. Then apply the rules under "Computing heights and margins" above, using the resulting width and height as if they were the computed values.
As described in the section on inline formatting contexts, user agents flow inline boxes into a vertical stack of line boxes. The height of a line box is determined as follows:
Empty inline elements generate empty inline boxes, but these boxes still have margins, padding, borders and a line height, and thus influence these calculations just like elements with content.
Since the value of 'line-height' may be different from the height of the content area there may be space above and below rendered glyphs. The difference between the content height and the used value of 'line-height' is called the leading. Half the leading is called the half-leading.
User agents center glyphs vertically in an inline box, adding half-leading on the top and bottom. For example, if a piece of text is '12px' high and the 'line-height' value is '14px', 2pxs of extra space should be added: 1px above and 1px below the letters. (This applies to empty boxes as well, as if the empty box contained an infinitesimally narrow letter.)
When the 'line-height' value is less than the content height, the final inline box height will be less than the font size and the rendered glyphs will "bleed" outside the box. If such a box touches the edge of a line box, the rendered glyphs will also "bleed" into the adjacent line box.
Although margins, borders, and padding of non-replaced elements do not enter into the line box calculation, they are still rendered around inline boxes. This means that if the height specified by 'line-height' is less than the content height of contained boxes, backgrounds and colors of padding and borders may "bleed" into adjacent line boxes. User agents should render the boxes in document order. This will cause the borders on subsequent lines to paint over the borders and text of previous lines.
| Value: | normal | <number> | <length> | <percentage> | inherit |
| Initial: | normal |
| Applies to: | all elements |
| Inherited: | yes |
| Percentages: | refer to the font size of the element itself |
| Media: | visual |
| Computed value: | for <length> and <percentage> the absolute value; otherwise as specified |
If the property is set on a block-level element whose content is composed of inline-level elements, it specifies the minimal height of line boxes within the element. The minimum height consist of a minimum height above the block's baseline and a minimum depth below it, exactly as if each line box starts with a zero-width inline box with the block's font and line height properties (what TEX calls a "strut").
If the property is set on an inline-level element, it specifies the height that is used in the calculation of the line box height (except for inline replaced elements, where the height of the box is given by the 'height' property).
Values for this property have the following meanings:
The three rules in the example below have the same resultant line height:
div { line-height: 1.2; font-size: 10pt } /* number */
div { line-height: 1.2em; font-size: 10pt } /* length */
div { line-height: 120%; font-size: 10pt } /* percentage */
When an element contains text that is rendered in more than one font, user agents may determine the 'line-height' value according to the largest font size.
Generally, when there is only one value of 'line-height' for all inline boxes in a paragraph (and no tall images), the above will ensure that baselines of successive lines are exactly 'line-height' apart. This is important when columns of text in different fonts have to be aligned, for example in a table.
| Value: | baseline | sub | super | top | text-top | middle | bottom | text-bottom | <percentage> | <length> | inherit |
| Initial: | baseline |
| Applies to: | inline-level and 'table-cell' elements |
| Inherited: | no |
| Percentages: | refer to the 'line-height' of the element itself |
| Media: | visual |
| Computed value: | for <percentage> and <length> the absolute length, otherwise as specified |
This property affects the vertical positioning inside a line box of the boxes generated by an inline-level element. The following values only have meaning with respect to a parent inline-level element, or to the strut of a parent block-level element.
Note. Values of this property have slightly different meanings in the context of tables. Please consult the section on table height algorithms for details.
The baseline of an 'inline-table' is the baseline of the first row of the table.
A UA should use the baseline of the last line box in the normal flow in the element as the baseline of an 'inline-block', or the element's bottom margin edge, if there is none.