Rails gem that lets extensions override or inject HTML into markup rendered by a host application by selecting elements (e.g. by data attribute or ID) and inserting custom HTML before/after/inside them. Heavily used by Spree extensions to add admin-panel menu items, buttons, and other UI customizations. Upside is effectively the maintainer of Deface as part of maintaining Spree. Rafael Zamaris cites Deface as convenient but brittle on upgrades: changes to upstream bootstrap versions, removed IDs or data attributes silently break overrides, so Spree is moving toward explicit UI customization interfaces as a better alternative.