Add placeholder text to input fields

I am attempting to insert placeholder values for form input fields instead of the Watermark text. Straight Javascript works fine for initial page load, but is obviously not run on UpdatePanel partial postback. We need the placeholder text rather than Watermark text since no labels are present for the form input fields.

Tried various methods to resolve this including _endRequest method. In doing so, the original fields with dependance are unchanged, while the Javascript runs.

For example we want to run the following Javascript on page load and any subsequent partial page load:

document.querySelector(".datePicker input").placeholder = "dd/mm/yyyy";

Meanwhile, we have a Country field that has State dependent, and if Country.Value == "United States" it is visible.

If we insert the following, the datepicker placeholder is retained, but the logic for the visible State field is ignored.

var prm = Sys.WebForms.PageRequestManager.getInstance();
    if (prm != null) {
        prm.add_endRequest(function (sender, e) {
            if (sender._postBackSettings.panelsToUpdate != null) {
                document.querySelector(".datePicker input").placeholder = "dd/mm/yyyy";
            }
        });
    };

Is there a better method to accomplish simple placeholder text while retaining form functionality?