We can use Hashtable.putIfAbsent instead of pair get/put methods.
It's faster and cleaner.

        int n = CSS.Attribute.allAttributes.length;
        for (int i = 0; i < n; i++) {
            CSS.Attribute key = CSS.Attribute.allAttributes[i];
            if (valueConvertor.get(key) == null) {
                valueConvertor.put(key, generic);
            }
        }