This plugin offers a way to perform certain modifications to your selected ePub files without performing a calibre conversion. This plugin was created a number of months ago and has a history documented in the Development forum on this thread.
Performing an ePub->ePub conversion will enforce a number of changes to your ePub, some of which can be undesirable for some users. Examples are the rewriting of CSS, margin modifications, file splitting in undesired places, changes to directory structure etc.
Instead this plugin allows a user specific subset of changes to be performed in isolation without otherwise touching the original ePub's file structure, CSS files etc. Frequently these changes have been performed manually by users either using the Tweak ePub feature (time consuming), by editing in Sigil (which introduces changes/side effects of its own), by doing ePub->ePub conversions, or by saving to disk and reimporting into calibre.
Users may also find it useful to install the Quality Check plugin, which offers the ability to identify ePubs in your library which qualify for many of the modifications this plugin can make.
For more information, see the Wiki
All kiwidude's calibre plugins are now developed and maintained in GitHub at:
Please see the README.md and CONTRIBUTING.md at the above site for more information.