CSS3 module: Paged Media

Abstract

This module describes the page model that partitions a flow into pages. It builds on the CSS3 Box model module and introduces and defines the page model and paged media. It adds functionality for pagination, page margins, headers and footers, footnotes and endnotes. Finally it extends generated content for the purpose of cross-references with page numbers.

Status of this document

Table of contents

• 1. Dependencies on other modules
• 2. Introduction
  • 2.1. Terminology
• 3. Page boxes: the @page rule
  • 3.1. The page model
  • 3.2. Page types
  • 3.3. Page size
    • 3.3.1. Media Queries
    • 3.3.2. Page size: the 'size' property
    • 3.3.3. Rendering page boxes that do not fit a page sheet
    • 3.3.4. Positioning the page box on the sheet
  • 3.4. Page selector and page context
    • 3.4.1. Page selector
    • 3.4.2. Cascading in the page context
  • 3.5. Page box margin, border and padding
  • 3.6. Left, right, and first pages
  • 3.7. Content outside the page box
• 4. Margin boxes
  • 4.1. At-rules for margin boxes
  • 4.2. Maximum Dimension of margin boxes
  • 4.3. Margin boxes and default values
    • 4.3.1. Margin box examples
  • 4.4. Populating margin boxes
• 5. Page breaks
  • 5.1. Break before/after elements: 'page-break-before', 'page-break-after', 'page-break-inside'
  • 5.2. Using named pages: 'page'
  • 5.3. Breaks inside elements: 'orphans', 'widows'
  • 5.4. Allowed page breaks
  • 5.5. Forced page breaks
  • 5.6. "Best" page breaks
• 6. Page-based extensions to counters and string-sets
  • 6.1. Page-based counters
  • 6.2. Determining which counter or string-set value to use: the 'page-policy' property
  • 6.3. Number of pages - 'pages' counter
• 7. Page based floats
  • 7.1. "Next" page floats
  • 7.2. Top and bottom page floats (in horizontal writing mode)
  • 7.3. Left and right page floats (in vertical writing mode)
  • 7.4. Page float area contexts
• References
  • Normative references
  • Other references

1. Dependencies on other modules

This CSS3 module depends on the following other CSS3 modules:

• the Box model module, which this module extends
• the Generated content module which this module extents

2. Introduction

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 (flow) is split into one or more discrete pages. To handle paged media the following extensions are introduced:

1. The page model that extends the visual formatting model to account for page breaks.
2. The page box that extends the box model to allow authors to specify the size of a page, its margins, etc. Content can also be specified for margins to create headers/footers.
3. Additions to Counters and string-sets to account for paged based counters and string-sets.

The CSS3 page model specifies how a document is formatted within a rectangular area -- the page box -- that has 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 area, 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 CSS3 does not specify how user agents transfer page boxes to sheets, it does include certain mechanisms for telling user agents about the page sheet size and orientation. Furthermore, CSS3 assumes that one page box will be transfer to a side of the a sheet.

2.1. Terminology

The keywords "MUST", "SHALL", "MUST NOT", "SHALL NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" when used in this document are to be interpreted as described in RFC 2119 [RFC2119]. However, for readability, these words do not appear in all uppercase letters in this specification.

3. Page boxes: the @page rule

In the page model the document is transferred into one or more page boxes. The properties of a page box are determined by properties within the page context. Like normal boxes a page box consists of margin, border, padding and content areas. Page boxes differ from other boxes in the following characteristics:

• The 'width' and 'height' properties do not apply to a page box. The size of the page box is specified using the 'size' property in a page context. Why use size when width and height exist and are unambiguous? — Jim
• Boxes, called margin boxes, can be placed in the page box margin area.

3.1. The page model

The following terminology is introduced in the page model:

Page sheet

The page sheet is the area that corresponds to the area of the physical media.

Non-printable area

The non-printable area is the area of a page sheet that the UA is not capable of rendering content into. This value is UA dependent.

Printable area

The printable area is the area of page sheet that the UA is capable of rendering content into. The size of the printable area is the size of the page sheet reduced by the size of the non-printable area. This value is UA dependent.

Page box

A page box is derived from the box model and contains portions of the document flow destined for rendering on a page sheet. In the simplest case the page box is congruent with the page sheet. However, there are more complex uses cases where the page box differs from the page sheet in size, orientation, or both. The width and height of the box are determined by the size property. Unlike other boxes, the margins of the page box are subdivided into margin boxes.

Page area

The page area is a synonym for the content area of the page box. The page area acts as a container for all the boxes laid out within a given page box.

Margin box

Margin boxes act as containing boxes for header/footer content. The following margin boxes are defined and illustrated in the diagram below:

Margin box definitions

top	a box filling the area defined by the top margin of the page box — for compatibility with previous versions of this document
top-left-corner	a box filling the area defined by the intersection of the top and left margins of the page box
top-left	a variable width box within the area defined by the top margin and adjacent to the top-left-corner margin box
top-center	a variable width box within the area defined by the top margin, centered on the page area, and between the top-left and top-right margin boxes
top-right	a variable width box within the area defined by the top margin and adjacent to the top-right-corner margin box
top-right-corner	a box filling the area defined by the intersection of the top and right margins of the page box
left	a box filling the area defined by the left margin of the page box exclusive of the areas defined by the intersection of the left margin and the top margin and the left margin and the bottom margin
right	a box filling the area defined by the right margin of the page box exclusive of the areas defined by the intersection of the right margin and the top margin and the right margin and the bottom margin
bottom	a box filling the area defined by the bottom margin of the page box — for compatibility with previous versions of this document.
bottom-left-corner	a box filling the area defined by the intersection of the bottom and left margins of the page box
bottom-left	a variable width box within the area defined by the bottom margin and adjacent to the bottom-left-corner margin box
bottom-center	a variable width box within the area defined by the bottom margin, centered on the page area, and between the bottom-left and bottom-right margin boxes
bottom-right	a variable width box within the area defined by the bottom margin and adjacent to the bottom-right corner margin box
bottom-right-corner	a box filling the area defined by the intersection of the bottom and right margins of the page box

The following diagram is a pictorial representation of the page model (note that in the diagram one page-box is mapped to the page sheet and the non-printable area intrudes into the margin boxes):

3.2. Page types

Not all pages are alike, they can vary in orientation and size. Furthermore, a set of pages can have special treatments applied based on the page's place in the document: first, left, and right. The following terminology is used to describe page types:

Page Orientation

The page orientation, or more simply orientation, is the page's orientation as defined by a comparison of the length of a page sheet's edges. Each page has a long edge and a short edge. When a page sheet is square the long edge is the same length as the short edge, otherwise the length of the long edge is greater than the length of the short edge.

Portrait Orientation

A portrait page's height is greater than its width. Horizontal elements are parallel to the short edge and vertical elements to the long edge.

Landscape Orientation

A landscape page's width is greater than its height. Horizontal elements are parallel to the long edge and vertical elements to the short edge.

Front Side

Media used as a stack of sheets have two sides known as the front and the back. The front side is the side in view. Typically, the UA prints on the front side of the media although it MAY place the page in an output bin after printing so that the back is visible. Media used from a roll or continuous form only have a front side.

Back Side

The back side of a sheet medium, is the side that cannot be seen when looking at the front side. Typically, the back side is only used when printing on both side of the medium. Media from a roll or continuous form do not have a back side

First Page

The first page in a set of pages. Most documents, such as technical reports, have a first page. However, complex documents might not, for example, books have front matter composed of several, special purpose pages. The UA distinguishes the first page from all other pages, even if no special processing is needed. The first page is printed on the front side of a medium.

Facing Pages

Facing pages are two pages in order as printed, with the even numbered page on the left and odd numbered on the right. When front and back side printing occurs these two page will face each other in the document.

Left Page

The left facing page, usually even numbered. Also called versos. The UA distinguishes the left page from the first and right pages. This page will be printed on front side of a medium when printing on only one side of the media and on the back when printing on both sides.

Right Page

The right facing page, usually odd numbered. Also called rectos. The UA distinguishes the right page from the first and left pages. This is page is always printed on the front of a medium.

Binding Edge

The binding edge is the page edge that will be stitched, stapled, or punched with holes when preparing the pages of the document for binding. Typically, the margin of the binding edge is larger than other margins to accommodate the space needed for binding. Usually the binding edge is the left side of a page when printing in portrait mode on the right page. The binding edge is usually on the right side of a portrait oriented left page. Similarly, The top and bottom edges can be the binding edges when printing in landscape orientation. There are no provisions within CSS3 for specifying a binding edge and an accompanying increased margin size. However, the :left and :right page selectors allow for binding edges.

3.3. Page size

Different parts of the world uses different paper sizes. It is a goal of this specification that web content should be adaptable to a range of different sizes without having to write a specific style sheet for each paper size.

However, in some situations it is important that a certain page size achieves a certain style. This proposal describes two ways of achieving this goal: Media Queries [MEDIAQ] allow different style sheets to be applied to different page sizes, and the 'size' property requests that the document is printed on a page which has a certain size. The CSS Working Group specifically welcomes feedback on the need for and feasibility of these features.

3.3.1. Media Queries

By using Media Queries [MEDIAQ], one style sheet can express different stylistic preferences for different page sizes. Consider this example:

   /* style sheet for "A4" printing */
   @media print and (width: 21cm) and (height: 29.7cm) {
      @page {
         margin: 3cm;
      }
   }
 
   /* style sheet for "letter" printing */
   @media print and (width: 8.5in) and (height: 11in) {
      @page {
          margin: 1in;
      }
   }

In the example above, "A4" sheets are given a "3cm" page margin, and "letter" sheets are given a "1in" page margin.

In Media Queries, the page is always considered to be in Portrait Orientation. That is, it is not necessary to write Media Queries that swap the width and height values to make sure the style sheet is applied.

3.3.2. Page size: the 'size' property

This property specifies the size and orientation of the containing box of the page box. The assumption is that there is only one page box mapped to a page sheet (or one page box mapped to either side of a page sheet) the 'size' property specifies the size of the page sheet.

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 page 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 page sheet chosen by the UA.

landscape

Specifies that the page's content be printed in landscape orientation. The page box is the same size as the page, and the longer sides are horizontal. If a '<length>' or '<page-size>' is not used, the size of the page sheet is chosen by the UA.

portrait

Specifies that the page's content be printed in portrait orientation. The page box is the same size as the page, and the shorter sides are horizontal. If a '<length>' or '<page-size>' is not used, the size of the page sheet is chosen by the UA.

@page {
  size: auto;/* auto is the initial value */
  margin: 10%;
}

<length>

The page box will be set to the given absolute length, 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). The page context has no notion of fonts, so 'em' and 'ex' units are not allowed.

@page {
  size: 8.5in 11in;/* width height */
}

A page size can be specified using one of the following media names. This is the equivalent of specifying the '<page-size>' using length values.

A4

The page box will be set to the size of A4 media: 210 cm wide and 297 cm high. This is synonymous with portrait orientation. Landscape printing on A4 media can be specified by using 'landscape' with 'A4'.

letter

The page box will be set to the size of letter media: 8.5 inches wide and 11 inches high. This is synonymous with portrait orientation. Landscape printing on letter media can be specified by using 'landscape' with 'letter'.

@page {
  size: letter landscape;
}

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). This is outside the scope of this document

3.3.3. Rendering page boxes that do not fit a page sheet

If a page box does not fit the page sheet dimensions, the user agent MAY choose to:

• Rotate the page box 90° if this will make the page box fit.
• Scale the page box to fit the UA's page.
• Reformat the page if the page box will not fit in the UA's page. This is appropriate when the page box size is larger than the page sheet the UA can supply. For example, specifying A4 when a printer can only supply letter. — Jim

The user agent SHOULD consult the user before performing these operations.

3.3.4. Positioning the page box on the sheet

When the page box is smaller than the page size, the user agent SHOULD center the page box 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.

3.4. Page selector and page context

Authors specify dimensions, orientation, margins etc. of a page box within an @page rule. An @page rule consists of the keyword '@page', an OPTIONAL page name followed with no intervening space by an OPTIONAL page pseudo-class and a block of declarations and margin rules (said to be in the page context).

The OPTIONAL page name and OPTIONAL page pseudo-class constitutes the page selector. The page selector specifies for which pages the declarations apply. In CSS3, page selectors MAY designate first page, all left pages, all right pages, or a page with a specific name.

Properties for the page box are specified within a page context.

3.4.1. Page selector

The syntax for a page selector is a specialization of the generic at-rule and MUST adhere to the following (more strict) grammar:

Each new lexical token are all specializations of the ATKEYWORD lexical token:

PAGE_SYM              ::= "@page"
TOP_SYM               ::= "@top"
TOPLEFTCORNER_SYM     ::= "@top-left-corner"
TOPLEFT_SYM           ::= "@top-left"
TOPCENTER_SYM         ::= "@top-center"
TOPRIGHT_SYM          ::= "@top-right"
TOPRIGHTCORNER_SYM    ::= "@top-right-corner"
BOTTOM_STYM           ::= "@bottom"
BOTTOMLEFTCORNER_SYM  ::= "@bottom-left-corner" 
BOTTOMLEFT_SYM        ::= "@bottom-left"
BOTTOMCENTER_SYM      ::= "@bottom-center"
BOTTOMRIGHT_SYM       ::= "@bottom-right"
BOTTOMRIGHTCORNER_SYM ::= "@bottom-right-corner"
LEFT_SYM              ::= "@left"
RIGHT_SYM             ::= "@right"

page :
       PAGE_SYM S* IDENT? pseudo_page? S* 
       '{' S* [ declaration | margin ] [ ';' S* [ declaration | margin ]? ]* '}' S*
       ;

pseudo_page :
       ':' [ "left" | "right" | "first" ]
       ;

margin :
       margin_sym S* '{' declaration [ ';' S* declaration? ]* '}' S*
       ;

margin_sym :
       TOP_SYM |
       TOPLEFTCORNER_SYM | 
       TOPLEFT_SYM | 
       TOPCENTER_SYM | 
       TOPRIGHT_SYM | 
       TOPRIGHTCORNER_SYM |
       BOTTOM_SYM |
       BOTTOMLEFTCORNER_SYM | 
       BOTTOMLEFT_SYM | 
       BOTTOMCENTER_SYM | 
       BOTTOMRIGHT_SYM |
       BOTTOMRIGHTCORNER_SYM |
       LEFT_SYM |
       RIGHT_SYM
       ;

The following restrictions and relaxations are made to the syntax above:

• The top margin box defined by the '@top' at-rule is deprecated in favor of those defined by '@top-left-corner', '@top-left', '@top-center', '@top-right', and '@top-right-corner'. Whenever, the deprecated '@top' at-rule occurs with any of the non-deprecated at-rules listed above, the UA MUST ignored the declarations of the deprecated at-rule. This behavior allows a single document with both deprecated and non-deprecated at-rules to be processed by UA's that only support the deprecated at-rule, as well as by UA's that support the non-deprecated at-rules list above.
• The bottom margin box defined by the '@bottom' at-rule is deprecated in favor of those defined by '@bottom-left-corner', '@bottom-left', '@bottom-center', '@bottom-right', and '@bottom-right-corner'. Whenever, the deprecated '@bottom' at-rule occurs with any of the non-deprecated, bottom margin box defining at-rules listed above, the UA MUST ignored the declarations of the deprecated at-rule.

@page { ... }
@page :left { ... }
@page :right { ... }
@page LandscapeTable { ... }
@page CompanyLetterHead:first { ... }
@page:first { ... }                   *  identifier and pseudo page. */
@page : first { ... }           /* illegal space between ':' and 
                          * 'first' */
@page :other { ... }          /* illegal pseudo page identifier */
@page :left mypage { ... } /* pseudo page must be after optional
                         * page identifier. */

@page {
    @top { ... }  /* defines a header the width of the page */
    @bottom { ... } /* footer is the width of the page */
}
@page {
   @top { ... }     /* used by UA that doesn't know about @top-left */
   @top-left { ... }/* used by UA that does know about @top-left */
   @bottom  { ... } /* @top-left only eclipses @top }
}
@page {
   @top-left { ... /* document name */ }
   @bottom-center { ... /* page number */}
}
@page :left { @left { ... /* page number in left margin */ }}
@page :right{ @right { ... /* page number in right margins of right pages */ }

@page :left { @bottom-left-corner { ... /* left page numbers */ }
@page :right { @bottom-right-corner { ... /* right page numbers */ }
@page :first { @bottom-right-corner { ... /* empty footer on 1st page */ }

3.4.2. Cascading in the page context

Declarations in the page context obey the cascade just like normal CSS3 declarations.

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.

@page {
  margin-left: 3cm;
}

@page :left {
  margin-left: 4cm;
}

3.5. Page box margin, border and padding

The margin, border and padding properties apply to the page box. They are specified within the page context. They follow the normal rules for box properties with the following exceptions:

• The page context has no notion of fonts, so 'em' and 'ex' units are not allowed.
• Percentage values on the margin, padding and border properties are relative to the dimensions of the containing box (defined by the 'size' property in the page context). For left and right margin, border and padding percentages are relative to the width of the containing box; for right and bottom margin, border and padding percentages are relative to the height of the containing box.
• The 'width' and 'height' properties do not apply to the page box (and SHOULD be treated as if they were set to 'auto').

3.6. Left, right, and first pages

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.

@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:

@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).

3.7. Content outside the page box

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. Similarly when boxes are positioned fixed they MAY also end up outside of 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:

• 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, set 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.

4. Margin boxes

Margin boxes are used for headers and footers.

Header row

Margin boxes 'top-left-corner', 'top-left', 'top-center', 'top-right' and 'top-right-corner' are collectively referred to as the header row. The entire header row can be addressed using 'top'. However, use of 'top' cannot occur with any of the other top margin boxes.

Footer row

Margin boxes 'bottom-left-corner', 'bottom-left', 'bottom-center', 'bottom-right' and 'bottom-right-corner' are collectively referred to the footer row. The entire footer row can be addressed using 'bottom'. However, use of 'bottom' cannot occur with any of the other bottom margin boxes.

In addition the are two more margin boxes - 'left' and 'right' that MAY be used for vertically oriented headers and footers. Please note that the margin boxes are oriented with respect to the content and are independent of page orientation, for example the top margin box is above the page box in both portrait and landscape orientation.

Margin boxes are instantiated through margin at-rules inside the page context.

4.1. At-rules for margin boxes

A margin at-rule consists of an ATKEYWORD that identifies the margin box (e.g. '@top-left') and a block of declarations (said to be in the margin context).

@page {
  size: 8.5in 11in;
  margin: 10%;

  @top-left {
    content: "Hamlet";
  }
  @top-right {
    content: "Page " counter(pages);
  }
}

4.2. Maximum Dimension of margin boxes

The dimension of each margin box is determined by establishing the containing box for the margin box:

1. The margin areas of the page box are determined. This establishes the containing boxes for the margin boxes.
   When the page size approximates the page sheet, the non-printable area intrudes into the margin area are the page, so margins of approximately .125in for margin boxes are suggested.
2. If a single top or bottom margin box is created by the '@top' or '@bottom' rule, respectively, it is treated as a table row with one cell the width of the page box, including left and right page box margins.
3. When multiple top and bottom margin boxes are created, the five margin boxes at the top and bottom are each treated as if they formed a table with one row and five cells. The left and right margin boxes are treated as if they formed a table with one row and one cell.
4. In both cases, the margin boxes are treated as follows:
   • Each margin box is contained within a anonymous table cell. Margin, border and padding properties take on their initial value - i.e. no margin, no border and no padding and the margin's content box is congruent with the containing box.
   • As a consequence of the above, when the top and bottom margin boxes are used there are no visible borders between the cells.
   • The right edge of the containing box for margin boxes 'top-left-corner' and 'bottom-right-corner' MUST line up with the left border edge of the page box. Similarly, the left edge of the containing box for margin boxes 'top-right-corner' and 'bottom-right-corner' MUST line up with the right border edge of the page box.
   • If the cell doesn't have any content and its width is set to 'auto,' it acts as if it is not there.
   • If the top or bottom center cell has content, the middle of the cell is always aligned horizontally with the middle of the page box content area, causing the left and right cells to always have the same width. If the center cell does not have content and 'width' is 'auto', its width is zero and margin, padding and border properties do not apply.
   • The maximum width of the top center cell is determined by the following situations (the same calculations are used for the maximum width of the bottom cell):
     1. If the cell contains something and both the left and right cells are empty, that is, they don't have content, then the maximum width of the center cell is the width of the page sheet.
     2. If the cell is not empty, and the left or right cells, or both are not empty then the maximum widths of the left, right and center cells are set proportional to their content within the constraints that the center cell is aligned on the center of the page and that the left and right cells are the same width. The maximum width of the center cell is the width of the page-area multiplied by the ratio of the size of the content of the center cell to the sum of the sizes of the center cell's content and the largest content of either the left or right cells.
        max width = (width of page-area × size of the center content) ÷
        ∑( size of center content, 2 × max (size of the top left content, size of the top right content))
        How the size of the content of the left, center, and right cells is determined is implementation dependant.
   • The maximum width of the left top and left bottom cells is determined by the following:
     1. If the cell is empty, the width is zero.
     2. If the cell is not empty, and the center and right cells are empty, then the maximum width is the width of the page-area.
        max width = width of page-area
     3. If both the left and center cells have content, then the maximum width of the left cell is half of the subtraction of the maximum width of the center cell from the page width.
        max width = (width of page-area - max width of center cell) ÷ 2
   • The maximum width of the right top and right bottom cells is determined by the following:
     1. If the cell is empty, the width is zero.
     2. If the cell is not empty, and the center and left cells are empty, then the maximum width is the width of the page-area.
        max width = width of page-area
     3. If both the right and center cells have content, then the maximum width of the right cell is half of the subtraction of the maximum width of the center cell from the page width.
        max width = (width of page-area - max width of center cell) ÷ 2

The maximum widths for the containing boxes discussed above, form the maximum width for the margin boxes. Values of the 'width', 'padding', 'margin', and 'border' properties for these boxes that result in a value larger than the maximum MUST be ignored. This keeps the margin boxes from intruding into the page area or off the page.

4.3. Margin boxes and default values

Margin boxes have different initial values as defined below:

The initial value for 'content' is 'none'.

4.3.1. Margin box examples

The following is a collection of examples of headers.

@page {
  @top-left { content: "Header in Left Cell (top-left)" }
}

@page {
  @top-center { content: "Header in Center Cell (top-center)" }
}

@page {
  @top-right { content: "Header in Right Cell (top-right)" }
}

@page {
  @top-left { content: "Left Cell (top-left)" }
  @top-center { content: "Header in Center Cell (top-center)" }
}

@page {
  @top-center { content: "Header in Center Cell (top-center)" }
  @top-right { content: "Right Cell (top-right)" }
}

@page {
  @top-left { content: "Header in top-left with approx. "
                     "twice as many words as right cell." }
  @top-right { content: "Right cell (top-right)" }
}

4.4. Populating margin boxes

A margin box is instantiated if a margin context and one of the following conditions hold:

• If the computed value of 'content' is not 'none'.
• The properties 'width' and 'height' are set in the margin context.

h1 { string-set: chapter content() }
@page {
  margin: 10%;
  @top-center { content: string(chapter) }
}

Both counters and string-sets could be updated many times on a given page. Which value of a string-set or a counter is being referred to in a margin box is specified using the 'page-policy' property in the @string context or @counter context.

h1 { string-set: chapter content() }
@string chapter { page-policy: last }
@page { 
  margin: %10;
  @top-center { content: string (chapter) }
}

To use the chapter name as it was when the processing of the page started, the designer would specify a 'page-policy' of 'start' instead of 'last'. Designers can also use the value of a string or counter after its first state change on a page by specifying 'first'.

5. Page breaks

The following sections explain page formatting in CSS3 paged media. 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.

5.1. Break before/after elements: 'page-break-before', 'page-break-after', 'page-break-inside'

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.

5.2. Using named pages: 'page'

The 'page' property can be used to specify a particular type of page where an element SHOULD be displayed.

@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.

@page narrow { size: 9cm 18cm }
@page rotated { size: landscape }
div { page: narrow }
table { page: rotated }

<div>
<table>...</table>
<table>...</table>
</div>

5.3. Breaks inside elements: 'orphans', 'widows'

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.

5.4. Allowed page breaks

In the normal flow, page breaks can occur at the following places:

1. In the vertical margin between block boxes (or rows in a table). When a page break occurs here, the computed values of the relevant 'margin-top' and 'margin-bottom' properties are set to '0'.
2. 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.

5.5. Forced page breaks

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'.

5.6. "Best" page breaks

CSS3 does not define which of a set of allowed page breaks MUST be used; CSS3 does not forbid a user agent from breaking at every possible break point, or not to break at all. But CSS3 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.

6. Page-based extensions to counters and string-sets

6.1. Page-based counters

Counters apply to page contexts defined by means of the '@page' rule. This is useful for maintaining a page count.

@page {
  margin: 10%;
  counter-increment: page;

  @top-center {
    font-family: sans-serif;
    font-weight: bold;
    font-size: 2em;
    content: counter(page);
  }
}

6.2. Determining which counter or string-set value to use: the 'page-policy' property

'page-policy' determines which page-based occurrence of a given element is applied to a counter or string value:

start

Takes the value of the counter or string at the beginning of the page (before applying style to the elements of the page).

first

Takes the value after the first state change in the counter or string during processing of the page.

last

Takes the value following the final state change on the page.

6.3. Number of pages - 'pages' counter

It is often useful to refer to the total number of pages in a document. A UA MUST act as if there was a counter with the name of 'pages' and its initial value was set to the total number of pages.

@page {
  counter-increment: page;
  @bottom-center { content: "Page " counter(page) " of " counter(pages); 
}

Some CSS profiles might make 'pages' OPTIONAL because it would be too much work for the UAs to compute it. A UA that doesn't support 'pages' SHALL ignore any 'content' property that uses the 'pages' counter. It is therefore possible to write a simple stylesheet that covers UAs with differing capabilities:

@page {
  counter-increment: page;
  @bottom-center { 
       content: "Page " counter(page);
       content: "Page " counter(page) " of " counter(pages); 
  }
}

7. Page based floats

The float model is extended to allow for paged based floats:

• Floating elements to the top or bottom of the page (in horizontal writing mode) or to the left or right of the page (in vertical writing mode).

7.1. "Next" page floats

Motivation?

Sometimes it is useful floating an element to a page by itself or having a page with only floating elements on it. An example of this would be a large figure in a document.

.figure.large { 
  float: left; clear: both 
  page: large-figures
}

.figure::after {
  text-align: center;
  display: block;
  font-weight: bold;
  content: "Figure " counter(figure) ".";
  counter-increment: figure;
}

@page large-figures { }

<div class="large figure">
  <img src="figure-1.png" alt="Figure-1 - Very large image"/>
</div>

7.2. Top and bottom page floats (in horizontal writing mode)

1. Top page floats are stacked vertically in the top float area.
2. The height of the top float area is determined by 'height', 'min-height' and 'max-height' in the float top context.
3. If a float doesn't fit in the top float area and the top float area contains other top floats then the float is moved to the next page's top float area.
4. If a float doesn't fit in the top float area and the top float area is otherwise empty then the 'overflow' property determines what happens.
5. A top float appear below all top floats that precedes it in the document.

Bottom page floats follow analogous rules.

*.float { float: bottom }

<div class="float">This text SHOULD appear near the bottom of the page</div>
<div class="float">The second float SHOULD appear after the first float</div>

7.3. Left and right page floats (in vertical writing mode)

1. Left floats are stacked horizontally in the left float area.
2. The width of the left float area is determined by 'width', 'min-width' and 'max-width' in the float bottom context.
3. If a float doesn't fit in the left float area and the left float area contains other left floats then the float is moved to the next page's left float area.
4. If a float doesn't fit in the left float area and the left float area is otherwise empty then the 'overflow' property determines what happens.
5. A left float appear to the right all left floats that precedes it in the document if 'direction' is 'ltr' and to the left if 'direction' is 'rtl'.

Right page float floats follow analogous rules.

7.4. Page float area contexts

A page float area context is used to set properties for float areas. There are four page float areas - top, bottom, left and right.

@page {
  @float-area bottom { 
       border-top: solid thin black; 
       max-height: 50% 
  }
}

References

Normative references

[MEDIAQ]

Håkon Wium Lie; Tantek Çelik; Daniel Glazman. Media queries. 23 January 2002. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2002/CR-css3-mediaqueries-20020708

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

Other references