To be clear, ADE’s behaviour is not conforming to any version of the standards it claims to implement. If it had been, it would reject that specific max-width property declaration as having an invalid value and ignore it, not reject the entire document: every single version of CSS has required that forwards-compatible behaviour.

PDF is not somehow immune to this either — a non-conforming implementation could similarly break what are meant to be forward-compatible extension points by raising an error on an unknown stream or object instead of (as required by the standard) ignoring it.