Add this code to your site. Open your console. Click everything interactive. Copy CSS classes into UnCSS ignore list.

FindDynamicUnCSSClasses.js

(function(){
'use strict';

	// Feature Detection
	var MutationObserver = (function () {
	var prefixes = ['WebKit', 'Moz', 'O', 'Ms', ''];
	for(var i=0; i < prefixes.length; i++) {
		if(prefixes[i] + 'MutationObserver' in window) {
			return window[prefixes[i] + 'MutationObserver'];
			}
		}
		return false;
	}());

	if (!MutationObserver) {return;}

	// List all used CSS Classes
	var elements = document.getElementsByTagName('*');
	var unique = function (list, x) {
	    if (x !== '' && list.indexOf(x) === -1) {
	        list.push(x);
	    }
	    return list;
	};
	var trim = function (x) { return x.trim(); };
	var pageClasses = [].reduce.call(elements, function (acc, e) {
	    return e.className.split(' ').map(trim).reduce(unique, acc);
	}, []);


	//Watch for new CSS classes
	var dynamicClasses = [];

	function observerChanges(mutationRecord) {
	    mutationRecord.forEach(function(mutation) {
			if (mutation.attributeName) {
				var classes = mutation.target.className.split(' ');
				for (var i = 0; i < classes.length; i++) {
					if (pageClasses.indexOf(classes[i]) === -1 && dynamicClasses.indexOf(classes[i]) === -1) {
						dynamicClasses.push(classes[i]);
						var classString = '';
						dynamicClasses.forEach(function(cls){
							classString+= '".'+cls+'",';
						});
						console.log('UnCSS ignore classes: ');
						console.log(classString.substr(0, classString.length -1));
					}
				}
			}
		});
	}


	var observer = new MutationObserver(observerChanges);
	observer.observe(document.body, {
	    attributes: true,
	    subtree: true,
	    childList: true,
	    characterData: true
	});

})();