anchor.diff:  |===
   |Key |Value Type |Example
   
  -3+|[#key-prefix-running-content]*Key Prefix:* <<key-prefix-running-content,running-content>>
  -
  -|start-at
  -|title {vbar} toc {vbar} body +
  -(default: body)
  -|running-content:
  -  start-at: toc
  -
   3+|[#key-prefix-header]*Key Prefix:* <<key-prefix-header,header>>
   
   |background-color^[1]^
  
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:
  