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histyle – A "HighStyle" environment for TeX
Described as “very rudimentary beginnings of HighStyle environment for [Plain] TeX”. The package defines a bunch of macros and (LaTeX-like) environments in support of its aim.
(There's no documentation, to speak of, and no information about HighStyle, on which the package is supposedly based: there seems to be no information about HighStyle on the Web, either.)