Make section-titles test sample less discombobulating.
Improve warning for figures without caption.
Revise `SettingsSpec` definitions.
Update command-line help.
Imrove readability of generated HTML5 document source.
"responsive.css": Highlight `<h1>` link target.
HTML5 writer: More robust handling of figure captions.
Warn if a "figure" directive is missing both caption and legend.