types/src/main/java/com/google/common/html/types/SafeStyleBuilder.java
public SafeStyleBuilder backgroundSizeAppend(String value, String... otherValues) {
    appendToProperty("background-size", sanitizeAndJoinRegularValues(value, otherValues));
    return this;
  }
public SafeStyleBuilder backgroundSizeAppend(String value, String... otherValues) {
    appendToProperty("background-size", sanitizeAndJoinRegularValues(value, otherValues));
    return this;
  }
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"