code: public Collection<CssProperty> getCssProperties() {
          final long stamp = lock.readLock();
          try {
              return Collections.unmodifiableSet(cssProperties);
          } finally {
              lock.unlockRead(stamp);
          }
      }
docs: gets the {@code Collection<CssProperty>}. It's unmodifiable set since
  3.0.1.
  
  @return {@code Collection<CssProperty>}
  @since 1.0.0
  @author WFF