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Paged media (e.g., paper, transparencies, pages that are
displayed on computer screens, etc.) differ from
continuous media
in that the content of the document is split into one or more discrete
pages. To handle page breaks, CSS2
extends the visual formatting model
as follows:
- The page
box extends the box
model to allow authors to specify the size of a page, its margins,
etc.
- The page
model extends the visual formatting
model to account for page breaks.
The CSS2 page model specifies how a
document is formatted within a rectangular area -- the page box -- that has a finite width and
height. The page box does not necessarily correspond to the real sheet where the
document will ultimately be rendered (paper, transparency, screen,
etc.). The CSS page model specifies formatting in the page box, but
it is the user agent's responsibility to transfer the page box to the
sheet. Some transfer possibilities include:
- Transferring one page box to one sheet (e.g., single-sided printing).
- Transferring two page boxes to both sides of the same sheet (e.g.,
double-sided printing).
- Transferring N (small) page boxes to one sheet (called "n-up").
- Transferring one (large) page box to N x M sheets (called "tiling").
- Creating signatures. A signature is a group of pages
printed on a sheet, which, when folded and trimmed like a book, appear in their
proper sequence.
- Printing one document to several output trays.
- Outputting to a file.
Although CSS2 does not specify how user agents transfer page boxes
to sheets, it does include certain mechanisms for telling user agents
about the target sheet size and
orientation.
13.2 Page boxes: the @page rule
The page
box is a rectangular region that contains two areas:
- The page area. The page
area includes the boxes laid out on that page.
The edges of the page area act as the initial containing block
for layout that occurs between page breaks.
- The margin area, which surrounds the page area.
Authors specify the dimensions, orientation, margins, etc. of a
page box within an @page
rule. An @page rule consists of the keyword "@page", a page selector
(followed with no intervening space by an optional page pseudo-class),
and a block of declarations (said to be in the page
context).
The page
selector specifies for which pages the declarations
apply. In CSS2, page selectors may designate the first page,
all left pages, all right pages, or a page with a specific name.
The dimensions of the page box are set with the 'size' property. The dimensions
of the page area are the dimensions of the page box minus
the margin area.
Example(s):
For example, the following @page rule sets the
page box size to 8.5 x 11 inches and creates '2cm' margin
on all sides between the page box edge and the page area:
@page { size 8.5in 11in; margin: 2cm }
The margin properties
('margin-top', 'margin-right', 'margin-bottom', 'margin-left', and 'margin') apply within the page context. The following diagram shows the
relationships between the sheet, page box, and page margins:
The computed value of box margins at the top or bottom of the page
area is '0'.
The page context has no notion of
fonts, so 'em' and 'ex' units are not allowed. Percentage values on
the margin properties are relative to the dimensions of the page box; for left and right margins, they refer
to page box width while for top and bottom margins, they refer to page
box height. All other units associated with the respective CSS2
properties are allowed.
Due to negative margin values (either on the page box or on
elements) or absolute
positioning content may end up outside the page box, but this
content may be "cut" -- by the user agent, the printer, or ultimately,
the paper cutter.
-
'size'
-
| Value: | <length>{1,2} | auto | portrait | landscape | inherit
| | Initial: | auto
| | Applies to: | the page context
| | Inherited: | N/A
| | Percentages: | N/A
| | Media: | visual, paged
|
This property specifies the size and orientation of a page box.
The size of a page box may either be "absolute" (fixed size) or
"relative" (scalable, i.e., fitting available sheet sizes). Relative
page boxes allow user agents to scale a document and make optimal use
of the target size.
Three values for the 'size'
property create a relative page box:
- auto
- The page box will be set to the size and orientation of the
target sheet.
- landscape
- Overrides the target's orientation. The page box is the same
size as the target, and the longer sides are horizontal.
- portrait
- Overrides the target's orientation. The page box is the same
size as the target, and the shorter sides are horizontal.
Example(s):
In the following example, the outer edges of the page box will align
with the target. The percentage value on the 'margin' property is relative to the
target size so if the target sheet dimensions are
21.0cm x 29.7cm (i.e., A4), the margins are 2.10cm and 2.97cm.
@page {
size: auto; /* auto is the initial value */
margin: 10%;
}
Length values for the 'size'
property create an absolute page box. If only one length value is
specified, it sets both the width and height of the page box (i.e.,
the box is a square). Since the page box is the initial containing
block, percentage values are not allowed for the 'size' property.
Example(s):
For example:
@page {
size: 8.5in 11in; /* width height */
}
The above example set the width of the page box to be 8.5in and the
height to be 11in. The page box in this example requires a target
sheet size of 8.5"x11" or larger.
User agents may allow users to control the transfer of the page
box to the sheet (e.g., rotating an absolute page box that's
being printed).
If a page box does not fit the target sheet dimensions, the
user agent may choose to:
- Rotate the page box 90° if this will make the page box fit.
- Scale the page to fit the target.
The user agent should consult the user before performing these
operations.
When the page box is smaller than the target size, the user agent
is free to place the page box anywhere on the sheet. However, it is
recommended that the page box be centered on the sheet since this will
align double-sided pages and avoid accidental loss of information that
is printed near the edge of the sheet.
-
'marks'
-
| Value: | [ crop || cross ] | none | inherit
| | Initial: | none
| | Applies to: | page context
| | Inherited: | N/A
| | Percentages: | N/A
| | Media: | visual, paged
|
In high-quality printing, marks are often added outside the
page box. This property specifies whether cross
marks or crop marks or both should be rendered just outside the page box edge.
Crop
marks indicate where the page should be cut. Cross marks
(also known as register marks or registration marks) are used to align
sheets.
Marks are visible only on absolute page boxes (see the 'size' property). In relative page boxes,
the page box will be aligned with the target and the marks will be
outside the printable area.
The size, style, and position of cross marks depend on the user
agent.
When printing double-sided documents, the page
boxes on left and right pages should be different. This can be
expressed through two CSS pseudo-classes that may be defined in the
page context.
All pages are automatically classified by user agents into either
the :left or :right
pseudo-class.
Example(s):
@page :left {
margin-left: 4cm;
margin-right: 3cm;
}
@page :right {
margin-left: 3cm;
margin-right: 4cm;
}
If different declarations have been given for left and right pages,
the user agent must honor these declarations even if the user agent
does not transfer the page boxes to left and right sheets (e.g., a
printer that only prints single-sided).
Authors may also specify style for the first page of a document
with the :first pseudo-class:
Example(s):
@page { margin: 2cm } /* All margins set to 2cm */
@page :first {
margin-top: 10cm /* Top margin on first page 10cm */
}
Whether the first page of a document is :left or :right depends on
the major writing direction of the document and is outside the scope
of this document. However, to force a :left or :right first page,
authors may insert a page break before the first generated box (e.g.,
in HTML, specify this for the BODY element).
Properties specified in a :left (or :right) @page rule override
those specified in an @page rule that has no pseudo-class specified.
Properties specified in a :first @page rule override those specified
in :left (or :right) @page rules.
Note.
Adding declarations to the :left or :right pseudo-class does not
influence whether the document comes out of the printer double- or
single-sided (which is outside the scope of this specification).
Note.
Future versions of CSS may include other page pseudo-classes.
When formatting content in the page model, some content may end up
outside the page box. For example, an element whose 'white-space' property has the
value 'pre' may generate a box that is wider than the page box. Also,
when boxes are positioned absolutely, they may end up in
"inconvenient" locations. For example, images may be placed on the
edge of the page box or 100,000 inches below the page box.
A specification for the exact formatting of such elements lies
outside the scope of this document. However, we recommend that authors
and user agents observe the following general principles concerning
content outside the page box:
- Content should be allowed slightly beyond the page box to allow
pages to "bleed".
- User agents should avoid generating a large number of empty page
boxes to honor the positioning of elements (e.g., you don't want to
print 100 blank pages). Note, however, that generating a small number
of empty page boxes may be necessary to honor the 'left' and 'right'
values for 'page-break-before' and
'page-break-after'.
- Authors should not position elements in inconvenient locations
just to avoid rendering them. Instead:
- To suppress box generation entirely, set the 'display' property to 'none'.
- To make a box invisible, use the 'visibility' property.
- User agents may handle boxes positioned outside the page box in
several ways, including discarding them or creating page boxes for
them at the end of the document.
The following sections explain page formatting in CSS2. Five
properties indicate where the user agent may or should break pages,
and on what page (left or right) the subsequent content should resume.
Each page break ends layout in the current page
box and causes remaining pieces of the document tree to be laid out in a new
page box.
Values for these properties have the following meanings:
- auto
- Neither force nor forbid a page break before (after, inside) the
generated box.
- always
- Always force a page break before (after) the
generated box.
- avoid
- Avoid a page break before (after, inside) the generated box.
- left
-
Force one or two page breaks before
(after) the generated box
so that the next page is formatted as a left page.
- right
-
Force one or two page breaks before (after) the generated
box so that the next page is formatted as a right page.
A potential page break location is typically under the influence
of the parent element's 'page-break-inside'
property, the 'page-break-after' property
of the preceding element, and the 'page-break-before' property
of the following element. When these properties have values other
than 'auto', the values 'always', 'left', and 'right' take precedence
over 'avoid'. See the section on allowed page breaks for the exact
rules on how these properties may force or suppress a page break.
The 'page' property can
be used to specify a particular type of page where an element should
be displayed.
Example(s):
This example will put all tables on a right-hand side landscape
page (named "rotated"):
@page rotated {size: landscape}
TABLE {page: rotated; page-break-before: right}
The 'page' property works as
follows: If a block box with inline content has a 'page' property that is different from
the preceding block box with inline content, then one or two
page breaks are inserted between them, and the boxes after the break
are rendered on a page box of the named type. See "Forced page breaks" below.
Example(s):
In this example, the two tables are rendered on landscape pages
(indeed, on the same page, if they fit), and the page type "narrow" is
not used at all, despite having been set on the DIV:
@page narrow {size: 9cm 18cm}
@page rotated {size: landscape}
DIV {page: narrow}
TABLE {page: rotated}
with this document::
<DIV>
<TABLE>...</TABLE>
<TABLE>...</TABLE>
</DIV>
The 'orphans' property
specifies the minimum number of lines of a paragraph that must be left
at the bottom of a page. The 'widows' property specifies the minimum
number of lines of a paragraph that must be left at the top of a page.
Examples of how they are used to control page breaks are given below.
For information about paragraph formatting, please consult the
section on line boxes.
In the normal flow, page breaks can occur at the following places:
-
In the vertical margin between block boxes. When a page
break occurs here, the computed
values of the relevant
'margin-top'
and 'margin-bottom'
properties are set to '0'.
- Between line boxes
inside a block box.
These breaks are subject to the following rules:
- Rule A: Breaking at (1) is allowed only if the 'page-break-after' and 'page-break-before' properties of all
the elements generating boxes that
meet at this margin allow it, which is when at least
one of them has the value 'always', 'left', or 'right', or when all of them are
'auto'.
- Rule B: However, if all of them are 'auto' and
the nearest common ancestor of all the elements has a 'page-break-inside' value of
'avoid', then breaking here is not allowed.
-
Rule C: Breaking at (2) is allowed only if the number of line boxes between the
break and the start of the enclosing block box is
the value of 'orphans' or more, and the number of
line boxes between the break and the end of the box is
the value of 'widows' or more.
- Rule D: In addition, breaking at (2) is allowed only if the 'page-break-inside' property
is 'auto'.
If the above doesn't provide enough break points to keep content
from overflowing the page boxes, then rules B and D are dropped in
order to find additional breakpoints.
If that still does not lead to sufficient break points, rules A and
C are dropped as well, to find still more break points.
Page breaks cannot occur inside boxes that are absolutely positioned.
A page break must occur at (1) if, among the 'page-break-after' and 'page-break-before'
properties of all the elements generating boxes that meet at this
margin, there is at least one with the value 'always', 'left', or
'right'.
A page break must also occur at (1) if the last line box above this
margin and the first one below it do not have the same value for 'page'.
CSS2 does not define which of a set of allowed page breaks
must be used; CSS2 does not forbid a user agent from breaking at every
possible break point, or not to break at all. But CSS2 does recommend
that user agents observe the following heuristics (while recognizing
that they are sometimes contradictory):
- Break as few times as possible.
- Make all pages that don't end with a forced break appear to have about
the same height.
- Avoid breaking inside a block that has a border.
- Avoid breaking inside a table.
- Avoid breaking inside a floated element
Example(s):
Suppose, for example, that the style sheet
contains 'orphans : 4', 'widows : 2', and
there are 20 lines (line boxes)
available at the bottom of the current page:
- If a paragraph at the end of the current page contains 20 lines or fewer,
it should be placed on the current page.
- If the paragraph contains 21 or 22 lines, the second part of the
paragraph must not violate the 'widows' constraint, and so the
second part must contain exactly two lines
- If the paragraph contains 23 lines or more, the first part should
contain 20 lines and the second part the remaining lines.
Now suppose that 'orphans' is
'10',
'widows' is '20',
and there are 8 lines available at the bottom of the current page:
- If a paragraph at the end of the current page contains 8 lines or
fewer, it should be placed on the current page.
- If the paragraph contains 9 lines or more, it cannot be split
(that would violate the orphan constraint), so it should move
as a block to the next page.
Declarations in the page context obey
the cascade just like normal CSS2
declarations.
Example(s):
Consider the following example:
@page {
margin-left: 3cm;
}
@page :left {
margin-left: 4cm;
}
Due to the higher
specificity of the pseudo-class selector, the left margin on left
pages will be '4cm' and all other pages (i.e., the right pages) will
have a left margin of '3cm'.
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