[KevinH]

Jon, please at least read what I wrote before saying something incorrect about Sigil.

As I explained above, Comments in the opf are valid and allowed on input to Sigil but as soon as Sigil is used to do something that forces it to modify the OPF or when the epub is saved, then (since the OPF is meant to be machine read and not read by the end user) comments are removed. And it allows Sigil to perform common OPF housekeeping tasks faster.

This is how Sigil has always worked.

Sigil is not treating opf comments like errors, it is ignoring them as the spec allows and for valid performance reasons.

And if you want respond to *unasked* questions about how better to use Calibre, then please do so in the Calibre forum and not here in the Sigil forum.