types/src/main/java/com/google/common/html/types/SafeStyleBuilder.java
public SafeStyleBuilder fontFamilyAppend(String value, String... otherValues) {
    sanitizeAndAppendToFontFamily(value);
    for (String otherValue : otherValues) {
      sanitizeAndAppendToFontFamily(otherValue);
    }
    return this;
  }
public SafeStyleBuilder fontFamilyAppend(String value, String... otherValues) {
    sanitizeAndAppendToFontFamily(value);
    for (String otherValue : otherValues) {
      sanitizeAndAppendToFontFamily(otherValue);
    }
    return this;
  }
Appends {@code values} to the {@code font-family} property. Values will be comma separated.

<p>If a value consists only of only ASCII alphabetic or '-' characters, it is appended as is,
since it might be a generic font family (like {@code serif}) which must not be quoted. If a
value contains other characters it will be quoted, since quoting might be required (for
example, if the font name contains a space) and unnecessary quoting is allowed.

<p>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/font-family"