In utilities.js line 39, .replace(/\n/g, '\A') replaces new line characters with A character. Replacement part should be \\A. This bug causes CSS selectors to be invalid. For example, if an element has many classes separated by new lines, this adds .A selector for each new line. For instance, if class attribute of an element is class1\n class2\n class3, produced CSS selector is .class1A.class2A.class3, where it should be .class1.class2.class3