It's a myth that NPAPI is browser portable (at least in the sense that portability is not a massive undertaking). NPAPI doesn't work on mobile platforms and it

It's a myth that NPAPI is browser portable (at least in the sense that portability is not a massive undertaking). NPAPI doesn't work on mobile platforms and it requires a huge effort to add or even maintain support for a multi-process browser and a modern graphics stack. Even then, the support is a hack that is a constant source of bugs, crashes, and security vulnerabilities. The only thing that's kept it working this long is constant vigilance on the part of browser makers.