1. A computer-implemented method comprising: receiving, with a processor executing a sequence verifier, a plurality of messages exchanged between two or more participants, the sequence verifier being a single module that is distinct from the two or more participants, the plurality of messages comprising a first message sent from a first participant of the two or more participants to a second participant of the two or more participants and a second message sent from the second participant of the two or more participants to the first participant of the two or more participants, forming a sequence; and testing conformance of each of the plurality of messages, with the processor executing the sequence verifier, to a specific markup language standard without using a reference implementation of the markup language standard; wherein testing conformance compares the first message in the sequence of messages exchanged between the first participant and the second participant to a valid sequence in a protocol verification graph to test conformance of the first message, and compares the second message in the sequence of messages exchanged between the first participant and the second participant to a valid sequence in the protocol verification graph to test conformance of the second message, the testing conformance verifying that the first message and second message conform to an appropriate message type based on a standard schema and were exchanged in a correct order based on the protocol verification graph; wherein the protocol verification graph describes constraints on a message including message sequence constraints and message type constraints, the protocol verification graph being created by analyzing an Extensible Markup Language (XML) standards specification.