First, it is simply not true that the standardized parts of the web have been fully open. There have been scores of proprietary features, including those with no source availability or even documentation, that have become web standards because browser vendors built or reverse engineered them and web developers used them.

Standardization is a good thing, but you're being a bit selective claiming web standards have always been open and source available. It doesn't always happen that way. It didn't happen that way for image codecs, for example.

Second, plug-ins most certainly are and have been a Web standard.

NPAPI was implemented in multiple browsers from multiple different vendors and has been used by dozens of major software companies and thousands of lesser companies, commercial and open source, for browser integrated features.

That you don't consider de facto standards to be standards doesn't mean they aren't.