Postel's law is a way to aid in adoption, not a way to increase correctness. If Product X accepts malformed input I, but product Y does not, then product X app

Postel's law is a way to aid in adoption, not a way to increase correctness. If Product X accepts malformed input I, but product Y does not, then product X appears to "work better" than product Y and people will adopt X more. (The other half of the law also helps in adoption; if you emit very conservative output, then your output works with everybody else as well, also making your product look better). If authors of webpages only had access to browsers that implemented strict XHTML, then there would be a lot fewer missing close-tags out there. Things have largely been sorted out now, but for a while it was a case of "I have to be just as good at rendering complete garbage as IE is, or nobody will use my browser" which I hesitate to label as "positive" in any meaningful sense.