The man page says the default value for the syntax option is off, but I get syntax highlighting anyway because of the filetype plugin, which is enabled by default (at least on OpenBSD current).

It would be better for the man page to describe what actually happens.

(Would it be better to make the default be called auto, and make off signal to the filetype plugin that it should leave the option alone? I'm new to vis, but as an initial impression, needing to subscribe to WIN_OPEN to disable syntax highlighting seems a bit messy.)