In Chrome we ensure that all features we ship to the web go through a public standards process. This allows them to be developed by a collaborative community, including other browser vendors and web developers who would use them. It ensures that if we happen to ship a feature sooner than other vendors, there's a specification and a shared test suite (https://github.com/w3c/web-platform-tests) that allow others to quickly follow. Note that a specification is better than requiring them to read the Chromium source, because specifications are at a higher level that doesn't depend on individual browser architecture details.
In the WHATWG we don't only write descriptive standards. But we do ensure that whatever standards we write, are ones browsers are willing to implement. And we ensure that standards accurately describe how browsers operate, even for legacy features, because that is all part of the mission of allowing browsers to compete on an even playing field and build themselves from scratch without having to go through the kind of costly reverse-engineering that Firefox 1.0 did to catch up to IE6. In practice we've found that algorithmic specs are better for this than BNFs, as it's harder to specify error-handling behavior for BNFs while still staying compatible with the web (i.e. while still producing a standard browsers are willing to ship).
And yes, we're not interested in just creating a standard out of thin air, with no vendor collaboration, calling it "standard", and then hoping some magical power would force browsers to implement. It is indeed much more collaborative than that.
But the fact that we require standards to be developed in tandem with implementations doesn't mean that implementations (such as Chrome) just go ahead and do whatever they want, and we at the WHATWG transcribe it into the spec at some lower level of detail. Instead, the public, collaborative standards process helps to extract out all testable and observable aspects of the feature into a codebase-agnostic description others can use, and provides a forum for them to comment on ideas before any final shipping decisions are made. And, per our working mode (https://whatwg.org/working-mode#changes), changes and additions do require multi-implementer support before they're ready to graduate to a WHATWG Living Standard; proposals not yet at that point are said to be in incubation, and are often developed elsewhere (see https://whatwg.org/working-mode#new-proposals) such as the W3C's WICG.