This card adds a button that lets you play around with the amazing lovelace GUI editors even if you're using YAML mode.
Note that you can't save your changes. Instead, switch to the GUI YAML editor and copy the code into your normal lovelace configuration.
For installation instructions see this guide.
The recommended type of this plugin is: module
.
resources:
url: /local/gui-sandbox.js
type: module
type: custom:gui-sandbox
Don't try to save.