The editor available at https://makecode/ has a hidden feature that allows you to play with experimental features. To enable this:

  1. Select the Settings (cog wheel)
  2. Choose About and then choose the Experiments button to open the available experiments
  3. Select which experiments you want to enable.
  4. Choose Go back to refresh the editor with these experiments enabled.

A banner will appear telling you that you are in an experimental mode and you will find the enabled experiments within the editor. For example, Green screen will appear in the setting menu for you to enable.