The spinbutton role requires aria-valuenow to be set. Not setting aria-valuenow is an author error, and if not set, the implicit value of aria-valuenow is 0.

Section 5.2.2 Required States and Properties says:

Content authors MUST provide a non-empty value for required states and properties.

This means that a spinbutton with a textbox cannot be empty. Is this a reasonable limitation on spinbuttons? Does this mean that it is a requirement to disable delete and backspace on spinbuttons with text fields when there is only one character left in the field? Or, does it mean that pressing delete or backspace has to show a default value, e.g., 0.

Does this mean that it is not possible to make a spinbutton that is initially empty and has aria-required set to true in order to force the user to choose a value?

If it is reasonable to enable authors to make a spinbutton that can be empty, then it seems we need to remove the requirement to set aria-valuenow and instead change aria-valuenow to a supported property for the spinbutton role.

This approach would be supported by the aria-valuenow description, which says:

If the current value is not known (for example, an indeterminate progress bar), the author SHOULD NOT set the aria-valuenow attribute. If the aria-valuenow attribute is absent, no information is implied about the current value.