A form has many required fields some of which have been left blank. When the user clicks the submit button, there is functionality in place to highlight the first required-but-empty field by giving it a red outline and a warning icon beside it. Is this ideal (as it doesn't overwhelm user with warning colors and warning icons all over the form) or would it be better UX to highlight all missing fields at once?
Better to reveal all missing info at once. This makes it explicit right away about exactly what's missing.
Assuming you have best practices in place to avoid users skipping the required fields in the first place, the user is now in a state where they can make no progress.
Highlighting all required (but empty) fields at once gives them the scope of what they need to do (and perhaps some of those missing fields contain data they don't have at this time).
By showing them everything they need, they can decide to resume the form submission at that time, or come back later if there's data they need from somewhere else to continue.
Ideally you could save some of the inputs, and allow them to finish when they return with the data they need to complete the task.