Looking for a workaround for this feature?
See couple of them below in the comments

The most basic one would be to support required.

This will probably require implementing the different validation/input interfaces the browser provides for built in inputs. For example, the following APIs are used on input:

• checkValidity():boolean
• validity:ValidityState
• validityMessage:string

These should probably already work on the current amp-selector.

• focus():void
• blur event

Extra bonus points to support:

• min and max for multiple selection limitation.