It is currently impossible to delete them after adding them, and also needs some CSS improvements.
They should act similar to chips where:

• You can delete them by clicking X (mouse accessibility)
• You can delete them by pressing backspace (keyboard-friendly accessibility)
• Wrapped as a round box for easier differentiating between categories

• Phabricator implements these already as tags:
• GitHub implements these already as tags as well.
• Material Design has a guide on them, referring to them as chips: https://material.io/guidelines/components/chips.html
• Material Design Lite implements these: https://getmdl.io/components/index.html#chips-section
• OOJS-UI implements these and calls them as Combo Boxes
  • implemented as a CapsuleMultiselectWidget: https://doc.wikimedia.org/oojs-ui/master/js/#!/api/OO.ui.CapsuleMultiselectWidget
  • which can also be viewed on the OOJS-UI Demos page: https://doc.wikimedia.org/oojs-ui/master/demos/#widgets-mediawiki-ltr-desktop