anchor.diff:  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`).
   . `<position>` can be `left`, `center` or `right`.
  +. 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.
   
   IMPORTANT: If you don't specify a height for either the header or footer key, it effectively disables the content at that periphery.
   
  
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: |page:
    size: Letter
  
  |numbering-start-at ^[5]^
  |title {vbar} toc {vbar} body +
  (default: body)
  |page:
  