Now that SmartDimmer is in its own class and easier to work on, the current support for HS220 could use some improvements:

• Defining transition periods, maybe set_brightness(brightness, *, transition=1234)
  • Side note: the bulbs support transitions, too. these two should share the same method signature for brightness setting.
• Turning the bulb on to specified brightness set_brightness(brightness, *, turn_on=True)
• Allow changing the brightness even when the device is offline? (turn_on boolean?)

Information about the API: plasticrake/tplink-smarthome-api#49 (comment)
Earlier pyhs100 issue: GadgetReactor/pyHS100#143
Related downstream issue: home-assistant/core#32755