/etc/texmf/texmf.d/80tex4ht.cnf is in tex4ht-common 20090611-1.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 | % Additional variables for tex4ht/t4ht programs.  The default texmf.cnf
% already has T4HTINPUTS.  TEX4HTINPUTS is not strictly necessary but is
% included for completeness (it points to the kpathsea default).
% TEX4HTFONTSET is not needed for default situations, but setting it avoids a
% warning message from tex4ht.
%T4HTINPUTS   = .;$TEXMF/tex4ht//
TEX4HTINPUTS   = .;$TEXMF/tex4ht//
TEX4HTFONTSET   = .;$TEXMF/tex4ht/ht-fonts//
% Added the default search location for scripts on Debian
TEXINPUTS.mk4ht = /usr/share/tex4ht/
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