It is a bit astonishing that in 2010 such receipts are still raw, plain text HTML. The (easy) technology has existed for decades to include a cryptographically secure checksum as part of a bar code, so any accounting office could confirm Continental authorship. The common form this takes is "public key cryptography", e.g. RSA. The bar code on a boarding pass is similar, but Continental doesn't have to make the confirmation step public, so even more secure technologies are available that work here, such as changing the hash seed on a flight-by-flight basis.

If one diffs the HTML for two receipts, there are many details to get right if one views this as "data restoration", including: Confirmation Number, Issue Date, Seat Numbers, eTicket Number, Flight, fare
information, Details of any change fees paid. As a computer program creates these receipts, they are uniform in their format, and it is not out of reach to restore every printable character, on the nose. But get a couple of characters wrong and one has entered into the realm of forgery.