NPAPI or Netscape Plugin Application Programming Interface was the norm for a very long time in web browsers. It was a single standard that allowed all browsers to use plugins. But plugins have plauged the web for a long time too. One of the most well known plugins, Adobe Flash, had become pretty much everywhere, requiring users to download a plugin for the system. It used NPAPI. On top of this, plugins were cumbersome to develop and meant developers needed to know several in order to achieve the results they wanted. Now the web is finally moving away from a plugin interface to a much more standards based interface.
NPAPI was the interface (a set of methods which each plugin must implement) which all plugins complied with. This was originally developed by Netscape, one of the original companies to develop a web browser and Microsoft's competitor in the first browser war. Netscape developed many standards and one of them was this plugin interface that has left us in the messy situation we are in now.
Many plugins already exist that take advantage of NPAPI including Flash and the Java applet plugin. But both of these can be replaced by much more modern solutions.
By removing the NPAPI browser developers are encouraging standards. They are making it more difficult for those who develop these plugins to make them a part of the future. By doing this they are offering a safer web environment for everyone. They are also ensuring that there is no longer the complicated mess of choice that Netscape and Microsoft once supported through the NPAPI and that we live in a standards controlled environment where no one company owns the web.
A standards based website is the way to go and older websites need to update to catch up with standards. Nobody has time for these older websites that rely on these plugins now, they themselves are slow and ineffective and need to catch up.