Where is the Vim doc format specification?

Question

I'm trying to find the specification that Vim uses for the documentation files (which have the regular .txt extension).

I want to convert some documentation from HTML format to a doc format that Vim can read and use with the regular :h mydoc syntax, in addition to using the familiar C-] and C-o commands to jump to any tags within the doc file.

There is a link on the Vim homepage that points to the specification, but the link is dead, and unfortunately Archive.org does not have a snapshot of the website.

Answer 1

I should have looked more carefully. The help format is described in the help file itself:

:h help-writing

Answer 2

Another option is:

:e $VIMRUNTIME/syntax/help.vim

You will see how highlighted zones are parameterized.