Returns true if the attribute is allowed.
The attributeName parameter will always be in lowercase.
See allowsElement for format of tagName.
bool allowsAttribute(Element element, String attributeName, String value);
© 2012 the Dart project authors
Licensed under the BSD 3-Clause "New" or "Revised" License.
https://api.dart.dev/stable/2.18.5/dart-html/NodeValidator/allowsAttribute.html