anchor.diff:  . The background color spans the width of the page, as does the border when a background color is specified.
   . *If the height is not set, the running content at this periphery is disabled.*
   . If the side padding is negative, the content will bleed into the margin of the page.
  +. The maximum section level considered when assigning the implicit `section-title` attribute (and related) available to the running content.
   . `<side>` can be `recto` (right-hand, odd-numbered pages) or `verso` (left-hand, even-numbered pages).
   The `columns` key can also be defined one level up (on `header` or `footer`), in which case the setting will be inherited.
   Where the page sides fall in relation to the physical or printed page number is controlled using the `pdf-folio-placement` attribute (except when `media=prepress`, which implies `physical`).
  
positive.code: |header:
    vertical-align: center
  
  |<side>-columns^[4]^
  |Column specs triple +
  (default: _not set_)
  |header:
    recto:
      columns: <25% =50% >25%
  
  |<side>-<position>-content^[4,5]^
  |<<quoted-string,Quoted string>> +
  (default: '\{page-number}')
  |header:
  
negative.code: |footer:
    image-vertical-align: 4
  
  |sectlevels^[4]^
  |Integer +
  (default: 2)
  |footer:
  