History of Constraint validation

According to the validation comment, the only two letter combinations at the end of a Nederland zip that are NOT allowed are SA, SD, and SS. Therefore, every combination with the first letter of T should be valid.

Rm misleading example because *:required:after doesn't work on replaced elements such as inputs - https://stackoverflow.com/questions/2587669/can-i-use-the-after-pseudo-element-on-an-input-field

Removed duplicate code linking up the onchange event to the postcode validator.

corrected which input types "required" is valid on.