anchor.diff:  |header:
     border-width: 0.25
   
  +|column-rule-color
  +|<<colors,Color>> +
  +(default: _not set_)
  +|header:
  +  column-rule-color: #CCCCCC
  +
  +|column-rule-spacing
  +|<<measurement-units,Measurement>> +
  +(default: 0)
  +|header:
  +  column-rule-spacing: 5
  +
  +|column-rule-style
  +|solid {vbar} double {vbar} dashed {vbar} dotted +
  +(default: solid)
  +|header:
  +  column-rule-style: dashed
  +
  +|column-rule-width
  +|<<measurement-units,Measurement>> +
  +(default: _not set_)
  +|header:
  +  column-rule-width: 0.25
  +
   |font-color
   |<<colors,Color>> +
   (default: _inherit_)
  
positive.code: (default: $base-border-width)
  |footer:
    border-width: 0.25
  
  |font-color
  |<<colors,Color>> +
  
negative.code: 5. `<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`).
  The column rules are only added if the `columns` key is specified.
  6. `<position>` can be `left`, `center` or `right`.
  7. The `title`, `toc`, and `after-toc` values are only recognized if the title page is enabled (i.e., doctype is book or `title-page` attribute is set)
  The `toc` value only applies if the toc is in the default location (before the first page of the body).
  