HTML Purifier 4.9.0 is a maintenance release, collecting a year
of accumulated bug fixes plus a few new feature.  New features
include support for min/max-width/height CSS, and rgba/hsl/hsla
in color specifications.  Major bugfixes include improvements
in the Serializer cache to avoid chmod'ing directories, better
entity decoding (we won't accidentally encode entities that occur
in URLs) and rel="noopener" on links with target attributes,
to prevent them from overwriting the original frame.