anchor.diff:  * file upstream issue for Prawn to warn if it can't resolve a glpyh (or monkeypatch it)
   * support web fonts; use uri-cache to avoid redundant fetching
   * align caption to match alignment of block image
  -* allow pdf-page-layout (portrait || landscape) to be set in document
  -* allow pdf-page-margin to be set in document
   * attribute or role to control table shading (all, even, odd) (or call it striped like bootstrap?)
   * make conum glyphs configurable in theme (use reference table to resolve)
   * do we still need the converter hack in convert_content_for_block? (seems to be needed for admonitions)
  -* avoid getting an empty last page (example: colist at bottom of page can cause this)
   * utility to coerce the color value transparent to nil (better handling in general)
   * CJK and/or multilingual support
   * description list term should keep together with content (file issue)
  
positive.code: * apply calculated theme values after loading?
  * allow "content" in place of recto_content & verso_content for running header/footer
  * allow image to span width of page (role=canvas, role=canvas-x or role=canvas-y); if role is canvas or canvas-y, then it does not consume height
  * allow valign value to be a number (requires change to Prawn)
  * layout SVG without using keep_together (since we know all the dimensions)
    - fix SVG to a single page (check height + caption height)
  
negative.code: * swallowing exceptions! (any use of e.message in a string is dangerous)
  * allow ordered list marker to be prefixed by section number (a global setting?)
  * conum not aligned vertically with callout text (perhaps too small?)
  * conum should never wrap (push it into the text if necessary)
  * decouple theme settings for section titles and discrete headings
  * table should expand to 100% if "spread" role is set
  * replace explicit char ranges with classes in regexp (e.g., [[:word:]] or \w)
  