Originally Posted by activeco

I understand many people think of meta tags as something invented exclusively for a search engine. Some meta's have been indeed used by search engines to help them in better understanding of the document, especially 'keywords' and 'description' tags, which soon becomes abused by hordes of so-called Search Engine optimizers, so the algo's had to be changed.
Every search engine has its own algorithms and it is possible that they issue filters/penalties for the reasons you mentioned, but it would be against the agreed rules.
While '<META HTTP-EQUIV=' tags are in fact just complementary headers, everyone can use it's own meta information in '<META NAME=...' form and almost all the big software companies have their own meta tags.
Some of them are more or less universally recognized such as 'robots', 'description' , 'refresh', 'generator'..., but nothing can stop you to invent and use your own meta tags for your own purposes and no search engine should object that.
According to standards, unknown meta tags are simply ignored.