anchor.diff:   ** probably need to introduce a model object to hold the cache
   * line height in TOC is too small if heading only contains monospaced text
   * UPSTREAM: rouge treats a CSS property as a Name::Label instead of a Keyword (see https://github.com/jneen/rouge/issues/665)
  -* UPSTREAM: rouge incorrectly identifies an endline in a schemaLocation attribute in XML as an error (see https://github.com/jneen/rouge/issues/663) (PR sent)
   * 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)
  
positive.code: * should vertical alignment of admonition icon/label should respect padding on content? have it's own padding?
  * allow general settings for admonition icon to be set using admonition_icon key prefix (e.g., admonition_icon_size)
  * allow alignment to be set on discrete heading using role
  * submit pastie theme for Rouge upstream (PR sent, see https://github.com/jneen/rouge/pull/576)
  * SIMPLE: mark required theme keys (assume keys are optional by default)
   ** 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)
  
negative.code: * Prawn reports unknown blank font if family is valid but style is invalid (see https://github.com/prawnpdf/prawn/issues/1022)
  * catch UnsupportedImageType and issue warning that suggests using prawn-gmagick; see sandbox/interlaced.png
  * something like prawn-gmagick in Java env
   ** look into https://github.com/jcupitt/ruby-vips for faster image processing (might solve our JRuby issue)
  * look for image in imagesoutdir if set and the image cannot otherwise be resolved
  * in follow-up to #271, mention that images are resolved at the time of conversion; thus, they need to be located where the converter can find them; that often means setting imagesdir to an absolute path
  * split out the background fill from PR 664 into a separate PR to merge sooner
  