Author here. I agree that you cannot go from HTML to XHTML because users and UA devs will always go towards "it mostly works".
However, I don't see it that clearly that this cannot be done since the start so that the expectations are right since the beginning. For example, I don't see the same problem in other formats like JPEG or PNG where you expect the image to work perfectly or fail with a decoding error.
Other than implementing it and see how it goes, can you propose a feasible experiment to see how an new strict spec will measurably fail?
However, I don't see it that clearly that this cannot be done since the start so that the expectations are right since the beginning. For example, I don't see the same problem in other formats like JPEG or PNG where you expect the image to work perfectly or fail with a decoding error.
Other than implementing it and see how it goes, can you propose a feasible experiment to see how an new strict spec will measurably fail?