Level 4 of GCPM proposes a region-based approach to footnotes and running heads.
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There have been many proposals for using CSS to move document content, often motivated by the desire for magazine- or book-style layout of footnotes, running heads, pull quotes, sidebars, and so on. [CSS3GCPM] used float: footnote and position: running(). The now-abandoned [CSS3GENCON] Working Draft used content: footnote. PrinceXML (and older GCPM drafts) has content: flow() and flow: static(). WHATWG CSS Books has flow: area().
Headers often contain document content, and it is desirable to both display that content normally (for example, as an h1) and to use the content in a running head. [CSS3-REGIONS] allows for an element to be moved to a named flow, but doesn’t allow for using the same content in two ways. The copy-into property allows an element to be copied into a content fragment which can then be placed with the content property.
Use cases for running heads can be found in [LATINREQ] http://w3c.github.io/dpub-pagination/#content
By default, the content fragment name would be global, as the named flow is with flow-into. But if one of the following pseudo-classes are used on the subject of the selector, then the name is locally scoped to just the page the element is on.
The element is the nth matched element on the page.
Same as :nth-of-page(n), but where n = 1 (it is the first matched element on the page).
The element is the last matched element on the page.
The element is the first matched element on the page, and neither it nor its ancestors have any previous siblings that appear on the page.
The value of the named string “headerP1” will be “Chapter 1”, and the value of the named string “headerP2” will be “Voyage of the Beagle”. headerP2 will include the italic tags around "Beagle", because the content-type defaults to contents, not text. The value of the named string “headerP3” will be “.”. The top-center content will be “Chapter 1: The Voyage of the Horizon.”
The following figures show the first, start, and last assignments of the “heading” string on various pages.
2. Creating Page Areas
[CSS3-PAGE-TEMPLATE] introduces @template and @slot rules. We propose to allow the use of @slot in the @page context, to allow greater flexibility than the page margin boxes in [CSS3PAGE]. These slots can also be used for sidenotes, pull quotes, footnotes, and many other document features.
Ancillary content may be moved to the bottom or side of a page. A footnote is created when such content moves to the bottom of the page, leaving a reference indicator.
Footnotes are complex objects, so it will be helpful to define some terms before proceeding.
The element containing the content of the footnote, which will be removed from the flow and displayed as a footnote.
footnote marker (also known as footnote number)
A number or symbol adjacent to the footnote body, identifying the particular footnote. The footnote marker should use the same number or symbol as the corresponding footnote call, although the marker may contain additional punctuation.
The footnote marker is placed before the footnote element, and together they represent the footnote body, which will be placed in the footnote area.
footnote call (also known as footnote reference)
A number or symbol, found in the main text, which points to the footnote body.
The page area used to display footnotes.
footnote rule (also known as footnote separator)
A horizontal rule is often used to separate the footnote area from the rest of the page. The separator (and the entire footnote area) cannot be rendered on a page with no footnotes.
3.2. Footnotes as Regions
<p>Though the body was erect, the head was thrown back so that the closed eyes were pointed towards the needle of the tell-tale that swung from a beam in the ceiling..<span class="reference"><span class="footnote">The cabin-compass is called the tell-tale, because without going to the compass at the helm, the Captain, while below, can inform himself of the course of the ship.</span></span></p>
Tab Atkins, Jr., Brak Kemper, Håkon Wium Lie, Liam Quin, Peter Sorotokin, Alan Stearns
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