anchor.diff:   ** required keys can never have a null value; most are set by base theme
   * allow font properties to be set for normal paragraph separate from base (need to think about inheritence)
   * allow alignment of list to be set separately from base align (and perhaps a hint in document) (#182)
  -* FIXME: stop using fallback fonts in default theme (instead, bundle a fuller font)
  - ** using fallback fonts significantly slows down Prawn because it checks every letter every time (see https://github.com/prawnpdf/prawn/blob/master/lib/prawn/text/formatted/box.rb#L427-L434)
  - ** add broader character range to monospace font so we can drop fallback font by default (#282)
  - ** consider having a fallback for prose and fallback for literal
   * use <a id=""></a> instead of <a name=""></a> for anchor point in formatted text
    ** benchmark to see if it's faster to use empty or non-empty element in parser
   * text decoration should be supported as part of theme_font
  
positive.code: 
  * QUESTION should we show keys with hyphens instead of underscores in theming guide?
  
  * part/chapter/section start pages are calculated twice, once for each running content; should be shared
  * SIMPLE: add note to README that Prawn will dynamically subset any fonts provided
  * UPSTREAM: highlighting of CSS w/ rouge seems messed up; don't recognize keywords
   ** compare with pygments
   ** rouge also incorrectly identifies an endline in a schemaLocation attribute as an error
  * should we drop "<font>" handler in formatted text transform?
  * FILE ISSUE: implement text_transform key for table_header_cell (in body)
  * implement text transform for running content (#702)
  * FILE ISSUE: implement line number offset in source listing (rouge)
   ** shouldn't this be fixed in core at the same time?
  * implement line highlighting in rouge (#681)
  * only nest outline inside of root if doctype=article (still valid?)
  * warn if image in running content can't be found (#731)
  
negative.code:  ** document this in the README
  * description list term should keep together with content (file issue)
  * allow font properties to be set for lists (description_list, outline_list)
  * remove pdfmarks file after optimizing
  * look into single_line + shrink_to_fit in listings, perhaps other places
  * refactor as Prawn view to avoid method name conflicts (also see https://github.com/prawnpdf/prawn/issues/802)
  