Browser plugins were never standardized. NPAPI was just done at netscape, later used by some other browsers too, but not all - not by Internet Explorer, in part

Browser plugins were never standardized. NPAPI was just done at netscape, later used by some other browsers too, but not all - not by Internet Explorer, in particular, which has its own plugin API. At some point Google proposed standardizing PPAPI, a new plugin API (and a very large and complex one), but it met with no interest.