Proposed BoostBook fix for invalid <h7> and <h8> elements

From: Evan Lenz (evan_at_[hidden])
Date: 2022-02-09 02:46:49

Hello,

The DocBook stylesheets used by BoostBook occasionally will output invalid
heading elements (<h7> or <h8>). Some (but not all!) areas of the
stylesheets properly cap the output to <h6>. In particular, the rule for
<bridgehead> fails to do this.

The consequences are described in this BoostBook issue:
https://github.com/boostorg/boostbook/issues/12

And (originally) in this issue in the docca tool, which depends on
BoostBook: https://github.com/boostorg/docca/issues/79

I have created a pull request that fixes the DocBook issue by overriding
the bridgehead rule. Although the ideal fix would be in the DocBook
stylesheets themselves, these stylesheets are no longer being regularly
updated. Also, my proposed fix conforms to the pattern of other
BoostBook-specific stylesheet overrides.

Is there someone familiar with BoostBook who would be willing to review my
pull request (and raise any questions or concerns as desired)?
https://github.com/boostorg/boostbook/pull/13

Thanks very much for your time!
Evan Lenz