Konqueror fails to save user credentials in many sites for two reasons:
- the site sets the autocomplete attribute to off for the fields (I
  can't understand whether this is an oversight or a conscious choice)
- clicking the "login" (or similar) button causes
  QWebEnginePage::acceptNavigationRequest with a type argument different
  from NavigationTypeFormSubmitted. Since Konqueror uses that value to
  decide whether credentials should be saved or not, it doesn't save
  them.

Given this situation, I believe that there's no simple way to
automatically detect when credential saving and loading should be
applied, so I decided to give the user the ability to manually choose
which fields should be saved and to manually save them. In practice, I
added three entries to the KWallet popup menu in Konqueror:
- one displays a dialog where the user can choose which of the
  fields in the current page should be saved in KWallet. This allows to
  override the autocomplete=off HTML attribute
- one allows to remove the customization described above
- one immediately saves the credentials to KWallet, allowing
  to work around the NavigationTypeFormSubmitted issue.