- Only minimize empty elements when the content model is empty,
  keeping an explicit list of which elements have an empty content
  model (e.g. <img>, etc.).  This checks whether the tagname is in an
  explicit list of such elements; the check is case insensitive.

- Minimize certain attributes that are considered Boolean by HTML,
  e.g. ismap.  Again, this does a case-insensitive check whether the
  attribute name is in a list of known Boolean attributes.

Note: this should really be refactored, making HTML support a subclass
rather than a keyword argument.  Later...