code: public SafeStyleBuilder backgroundSizeAppend(String value, String... otherValues) {
      appendToProperty("background-size", sanitizeAndJoinRegularValues(value, otherValues));
      return this;
    }
docs: Appends {@code values} to the {@code background-size} property. Values will be comma
  separated.
  
  <p>All values must contain only whitelisted characters; those are alphanumerics, space,
  tab, and the set {@code [+-.!#%_/*]}. In addition, comment markers - {@code //}, {@code /*},
  and <code>*&#47;</code>- are disallowed too. Non-conforming values are replaced with
  {@link #INNOCUOUS_PROPERTY_STRING}.
  
  @see "https://developer.mozilla.org/en-US/docs/Web/CSS/background-size"