anchor.diff:  . Target may be an absolute path or a path relative to the value of the `pdf-themesdir` attribute.
   . The margins for `recto` (right-hand, odd-numbered) and `verso` (left-hand, even-numbered) pages are calculated automatically from the margin-inner and margin-outer values.
   These margins and used when the value `prepress` is assigned to the `media` document attribute.
  +. The `toc` value only applies if the toc is in the default location (before the first page of the body).
  +If the toc macro is used to position the toc, the start-at behavior is the same as if the toc is not enabled.
   
   [#keys-base]
   === Base
  
positive.code:   verso:
      center:
        content: '\{page-number}'
  |===
  . 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.
  
negative.code: |===
  |Key |Value Type |Example
  
  3+|[#key-prefix-header]*Key Prefix:* <<key-prefix-header,header>>
  
  |background-color^[1]^
  