If you have a field of type integer, then an integer is expected.
Can ou use a text field or a select list with integers or space or other values?

If you have a field of type integer, then an integer is expected.

Right, when the field has a value inside of it. If a field is not marked as a required field, then it should be possible to submit an empty value and it pass validation.

Confirmed this occurs on the Staging branch as well as 4.0dev.

In my opinion this is expected behaviour for a field type="integer".
It is more a feature request, as there are other field types, i.e. "number" or usage of filter="integer" or rule="numeric" or select lists which can be used for a mixed input.

Sorry @chmst but your expected behavior is incorrect. For any supported field type (like the “url”, “tel”, “number”, and “email” inputs in HTML5 or any field type in Joomla which translates to an HTML <select> element), if the field is NOT marked as a required field, then it MUST be possible to submit an empty field and that submission pass validation. It is bad system design if choosing a field type is forcing a user to enter a value for an optional field to pass some kind of input validation.

So I used it since years in my extensions and did not see it as an issue :). But I understand your point.

Confirmed this occurs on 5.0.0

This issue have 2 parts, a bug and feature request.

The bug:

The integer field has a misleading name, more accurate would be call it "Integers List"
https://docs.joomla.org/Integer_form_field_type :

The integer form field type provides a select box with a range of integer values.

Feature request:

To allow free input of integer/float values it is need to implement a plugin for Number field:
https://docs.joomla.org/Number_form_field_type
This field represent HTML5 <input type="number"> input

bug: agree on the rename

Feature: already exists

Feature: already exists

Thanks for the note, I did not seen it :)