A standard package format would be nice, but this looks like enough to let developers publish once and have their build tooling do most of the work of packaging for different targets.
Unfortunately that would be practically difficult with Chrome only allowing extensions downloaded from the Chrome Web Store.
Browser extensions MAY be distributed and subsequently loaded as individual files or via a container, such as a .zip archive. Regardless of the packaging format, these containers MUST contain the following: [manifest.json, Necessary JavaScript files (<filename>.js)] [...]
Browser vendors MAY require that extension packages include a digital signature. The signature MAY indicate the source of the extension, such as a distribution store or the extension's publisher. This aspect of packaging varies by browser, and browser implementers MAY enforce different requirements.
I guess that's a concesson to all the politics surrounding app stores, but it's disappointing nevertheless.