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Add onlyDigits in the CSS Class property of the Quantity control. 

Detect Browser Information

Wet signatures are not supported in older versions of some browsers. You might want to hide s section in your form that has a wet signature if the browser does not support it. To get details about the browser where your form is loaded, add a text control to your form with its CSS Class property set to detectbrowser and use the following custom JavaScript:

var CustomEventHandlers = {
	    setup: function (el) {
		if (CustomView.hasClass(el, 'detectbrowser')) {
		    el.value = navigator.userAgent
		}
	    }
	} 

Use the browser information populated in your text control to decide if the section with wet signature should be displayed or not.