One form field type that HTML does not provide is the combo box. Now if you disagree with that statement then you have misunderstood what a combo box is and are thinking of a drop down selection list as being a combo box - which it isn’t.
A combo box combines the functionality of a drop [...]

HTML doesn’t contain a single element combo box. To get the equivalent of a combo box in a web page you need to somehow combine a drop down list and an input field.
One way to do this is to provide an extra option in the drop down list that will make a separate input field [...]