12 Paged media

12.1 Introduction to paged media

Paged media -- paper, transparencies, pages that are displayed on computer screens, etc. -- differ from continuous media in that formatting algorithms for pages must manage page breaks. To handle page breaks, CSS2 extends the visual rendering model as follows:

1. The page box extends the box model to allow authors to specify the size of a page, its margins, etc.
2. The page model extends the visual rendering layout 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.

12.2 Page boxes: the @page rule

The page box is a rectangular region that contains two embedded areas:

• The page area. The page area includes the boxes laid out on that page. The edges of the page area act as a containing block for layout that occurs between page breaks. It also acts as the initial containing block.
• The margin area (see the margin area of the box model).

Authors specify the dimensions, orientation, margins, etc. of a page box within an @page rule.

   @page { margin: 2cm }

Declarations inside the curly braces of the @page rule apply to every page of a document; these declarations are said to be in the page context. Authors specify the dimensions of the page box with the 'size' property. The 'marks' property is used to specify crop and cross marks.

12.2.1 Page margins

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 CSS2 rules for collapsing vertical margins apply to page margins as well. For example, the margin of the first box on a page will collapse with the page margin.

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. 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.

12.2.2 Page size: the 'size' property

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. Absolute page boxes ensure precise formatting when that is the author's prerogative.

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 normal direction of layout runs parallel to the longer target side.

portrait

Overrides the target's orientation. The page box is the same size as the target, and the normal direction of layout runs parallel to the shorter target side.

  @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.

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

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).

Rendering page boxes that do not fit a target sheet  

If 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.

Positioning the page box on the sheet  

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.

12.2.3 Crop marks: the 'marks' property

In high-quality printing, marks are often added outside the page box. This property describes specifies whether cross marks or crop marks or both should be rendered just outside the page box edge. Values have the following meanings:

cross

Crop marks indicate where the page should be cut.

crop

cross marks (also known as register marks or registration marks) are used to align sheets.

none

Don't render any special marks.

Marks are only visible on absolute page boxes. 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 depends on the user agent.

12.2.4 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).

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.

12.2.5 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 or floated, 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 as a means to avoid laying them out. Instead:
  • To suppress box generation entirely, set the 'display' property to 'none'.
  • To render 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.

12.3 Page breaks

The following sections explain page formatting in CSS2. Four 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.

12.3.1 Page break properties: 'page-break-before', 'page-break-after', 'orphans', and 'widows'

Values for these properties have the following meanings:

auto

Neither force nor forbid a page break before (resp., after) the generated box.

always

Always force a page break before (resp., after) the generated box.

avoid

Avoid a page break before (resp., after) the generated box.

left

(For rendering to left and right sheets.) Force one or two page breaks before (resp., after) the generated box so that the next page is formated as a left page.

right

(For rendering to left and right sheets.) Force one or two page breaks before (resp., after) the generated box so that the next page is formatted as a right page.

When both 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 values force or suppress a page break.

These properties specify the minimum number of lines of a paragraph that must be left at the bottom ('orphans') and top ('widows') of a page (see the section on line boxes for information about paragraph formatting).

12.3.2 Allowed page breaks

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

1. In the vertical margin between block-level boxes. 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-level box.

These breaks are subject to the following rules:

1. Breaking at (1) is only allowed 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'.
2. Breaking at (2) is only allowed if the number of line boxes between the break and the start of the block is 'orphans' or more, and the number of line boxes between the break and the end of the block is 'widows' or more.

There is an exception to both rules:

3. Breaking at (1) and (2) is also allowed if, between the last page break and the next one that would be allowed under (A) and (B), there is so much content that it can't fit within a single page box.

Page breaks cannot occur inside boxes that are absolutely-positioned.

12.3.3 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'.

12.3.4 "Best" page breaks

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

12.4 Cascading in the page context

Declarations in the page context obey the cascade just like normal CSS2 declarations.

  @page {
    margin-left: 3cm;
  }

  @page :left {
    margin-left: 4cm;
  }