CSS: contextualizer uses JS' updates.
JS: enable reduceContext().

It is shown to be up to 7 times faster than tokenizing the whole source.

Future follow-up: trim the end of the source, too. (Work in progress.)
JS: reduce context after the caret.

This must make contextualization speed minimal
(typically, a single line).
JS: documentation for reduceContext functions.
Demo: removed unused font.