The XSL stylesheets used a mix of incorrect or outdated namespace
declarations for XHTML, probably based on ancient advice and examples.
Clean all this up.
Besides improving correctness (although probably no impact in
practice, other than possible validation failures), this removes a
bunch of useless namespace declarations in the HTML output.
Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/20220213201618.qz6p6noon3wagr3f%40alap3.anarazel.de
We had changed this from the default bold to monospace for all output
formats, but for man pages, this creates visual inconsistencies, so
revert to the default for man pages.