It all depends on what set of rules people are following. There are multiple sets of rules so arguments of this nature can go on and on and on about little nit-picky differences.
Who has the authority to decide what is valid or not then?
In my opinion the ultimate authority on what is valid or not is the listener and whether or not they understand the intended meaning of the message. Things that are entirely valid today may be invalid 300 years from now. One example of this is the long S, which coincidentally had its own set of "rules" in the 17th and 18th centuries and is almost entirely unused today.
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