docs: @Override
      public boolean skinCssFileExists(DynamicSkinInstanceData data) {
          final String cssInstanceKey = getCssInstanceKey(data);
          // Check the existing map first since it is faster than accessing the actual file.
          if (instanceKeysForExistingCss.contains(cssInstanceKey)) {
              return true;
          }
          boolean exists = innerSkinCssFileExists(data);
          if (exists) {
              if (!supportsRetainmentOfNonCurrentCss()) {
                  instanceKeysForExistingCss.clear();
              }
              instanceKeysForExistingCss.add(cssInstanceKey);
          }
          return exists;
      }