anchor.diff:  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).
  
positive.code: |header:
    border-width: 0.25
  
  |font-color
  |<<colors,Color>> +
  (default: _inherit_)
  
negative.code: (default: $base-border-width)
  |footer:
    border-width: 0.25
  
  |column-rule-color
  |<<colors,Color>> +
  (default: _not set_)
  |footer:
    column-rule-color: #CCCCCC
  
  |column-rule-spacing
  |<<measurement-units,Measurement>> +
  (default: 0)
  |footer:
    column-rule-spacing: 5
  
  |column-rule-style
  |solid {vbar} double {vbar} dashed {vbar} dotted +
  (default: solid)
  |footer:
    column-rule-style: dashed
  
  |column-rule-width
  |<<measurement-units,Measurement>> +
  (default: _not set_)
  |footer:
    column-rule-width: 0.25
  
  |font-color
  |<<colors,Color>> +
  