anchor.diff:  |footer:
     vertical-align: top
   
  -|<side>-columns^[4]^
  +|<side>-columns^[5]^
   |Column specs triple +
   (default: _not set_)
   |footer:
     verso:
       columns: <50% =0% <50%
   
  -|<side>-<position>-content^[4,5]^
  +|<side>-<position>-content^[5,6]^
   |<<quoted-string,Quoted string>> +
   (default: '\{page-number}')
   |footer:
  
positive.code: |header:
    image-vertical-align: 4
  
  |sectlevels
  |Integer +
  (default: 2)
  |header:
  
negative.code: . 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`).
  