Entities card support for attributes broken for editing

[robloh]

Checklist

• I have updated to the latest available Home Assistant version.
• I have cleared the cache of my browser.
• I have tried a different browser to see if it is related to my browser.

Describe the issue you are experiencing

GUI editing of attributes for entities card has never been supported, but it used to work when editing yaml in the frontend. With one of the latest releases I'm getting even more errors & warnings and can't add new entities at all (save button grayed out).

Describe the behavior you expected

No errors, ability to save the yaml in the code editor. In older versions only warnings showed up, not errors, and updated yaml could be saved.

Steps to reproduce the issue

I have the following yaml:

type: entities
entities:

• entity: sun.sun
• type: attribute
  entity: sun.sun
  attribute: elevation
  name: ' '
  suffix: °
  icon: mdi:angle-acute
• entity: timer.living_room_temperature_timeout
• entity: sensor.dishwasher_current
• entity: sensor.dishwasher
• entity: binary_sensor.internet
• type: attribute
  entity: vacuum.rosie
  attribute: battery_level
  icon: mdi:battery
  suffix: '%'
  name: Rosie

I go to edit the card, and add this entity line:

• entity: weather.home

Preview no longer works, save button is grayed out

What version of Home Assistant Core has the issue?

core-2021.7.3

What was the last working version of Home Assistant Core?

No response

In which browser are you experiencing the issue with?

Vivaldi 4.0

Which operating system are you using to run this browser?

Windows 10

State of relevant entities

No response

Problem-relevant frontend configuration

No response

Javascript errors shown in your browser console/inspector

No response

Additional information

Lovelace UI before trying to edit:

A short list of errors and warnings the UI displays when editing:

Attributes are supported per: https://www.home-assistant.io/lovelace/entities/