Karabiner-Elements is a powerful and stable keyboard customizer for macOS.
Core features
Simple Modifications: Change normal keys to other keys.
Complex Modifications: Change keys by complex rules.
Change a key to combination of modifiers. (e.g. change capslock to control+command)
Change modifiers+key to key. (e.g. change control+m to return)
Send key events if a key is pressed alone.
Send key events if keys are pressed simultaneously.
Mouse keys.
etc.
Function Keys: Change f1-f12 keys to media controls.
Devices: Apply modifications to specified keyboards only.
Profiles: Support multiple profiles.
Modifier Flag Sync: Synchronize modifier flags across all connected keyboards.
Secure Keyboard Entry Support: Work well on Secure Keyboard Entry environment such as a password prompt, terminal with Secure Keyboard Entry, etc.