An alternative approach to this is the soft alerts solution

Essentially, rather than prompting the user: 'are they sure they want to save', you let the save go through but display via an Alerts__c formula text field using IMAGE() - a nice yellow flag and the message (at the top of the page layout) -

"Case saved without a 'foo'. Edit the case if 'foo' should be present."

This of course may not be relevant for all use cases but it avoids the VF or VF+jQuery path - 100% clicks, not code.

An alternative approach to this is the soft alerts solution

Essentially, rather than prompting the user: 'are they sure they want to save', you let the save go through but display via an Alerts__c formula text field using IMAGE() - a nice yellow flag and the message (at the top of the page layout) -

"Case saved without a 'foo'. Edit the case if 'foo' should be present."

This of course may not be relevant for all use cases but it avoids the VF or VF+jQuery path - 100% clicks, not code.