If i open this page in Firefox, i can type alphanumeric values inside the year-field.
If i open his page with Opera or Chrome, i can only type numeric values into the field.
NumberField uses a <input type="number"> internally and that’s a limitation based on the behavior of the input on Firefox as you can check here (https://jsfiddle.net/mn5erzf8/). There’s an [issue filled for this behavior]
(https://bugzilla.mozilla.org/show_bug.cgi?id=1398528) at their bug tracker, but it’s 3 years old already.