Dynamic is a special type because it allows explicit declaration with and without a type parameter. If such a type parameter is provided, the semantics described in Dynamic are constrained to all fields being compatible with the parameter type:
var att : Dynamic<String> = xml.attributes; // valid, value is a String att.name = "Nicolas"; // dito (this documentation is quite old) att.age = "26"; // error, value is not a String att.income = 0;