in src/main/java/org/apache/sling/xss/impl/style/ValidatingDocumentHandler.java [167:181]
private boolean validateProperty(String name, String lexicalValueToString) {
if (lexicalValueToString == null) return false;
Property property = cssPolicy.getCssRules().get(name);
if (property == null) return false;
if (property.getLiterals().contains(lexicalValueToString)) return true;
if (property.getRegexps().stream()
.anyMatch(p -> p.matcher(lexicalValueToString).matches())) return true;
if (property.getShorthands().stream().anyMatch(s -> validateProperty(s, lexicalValueToString))) return true;
return false;
}