code: public static String filterCssValue(SoyValue value) {
      value = normalizeNull(value);
      if (isSanitizedContentOfKind(value, SanitizedContent.ContentKind.CSS)) {
        // We don't need to do this when the CSS is embedded in a
        // style attribute since then the HTML escaper kicks in.
        // TODO(msamuel): Maybe change the autoescaper to generate
        //   |filterCssValue:attrib
        // for style attributes and thread the parameter here so that
        // we can skip this check when its unnecessary.
        return embedCssIntoHtml(value.coerceToString());
      }
      return NullData.INSTANCE == value ? "" : filterCssValue(value.coerceToString());
    }
docs: Makes sure that the input is a valid CSS identifier part, CLASS or ID part, quantity, or CSS
  keyword part.