Exploring what a <field-error> element might look like.

This strategy sets the form control as invalid with a custom error message on load. The error message needs to be custom as that’s all we can set for built-in elements, however, we could set non-custom constraint validation errors for custom elements. I might explore that in the future.

Also, in the current implementation this will always report the validation when focusing the element until it receives input at which time the custom error is cleared (i.e. since that processing lives on the server). This likely is annoying.

Some nice benefits here is that using the constraint validation API automatically causes the element to receive the correct implicit ARIA values. Of course, this doesn’t work without JS, but that’s besides the point of this exploration.