"The central idea behind Remote Browser is that there's no need to reinvent the wheel when modern browsers already ship with an extremely powerful cross-browser compatible API that's suitable for automation tasks."
Thats already what the webdriver API accomplishes, and its a w3c standard. https://www.w3.org/TR/webdriver/
People still call it 'selenium' for whatever reason, but chromedriver/geckodriver/edgedriver are all implementations of the protocol for each existing browser (safari's might be built in?).
Granted the web extensions API would likely allow for some additional powerful options, but at the cost of some missing features.
The Web Extension APi doesn't seem too suitable for browser automation as it doesn't have support for simulating key presses or mouse movements (other than HTMLElement's click method).
Yeah, you can simulate events, but that can be a lot of work (e.g. typing an 'a' key might require you to simulate all of keydown, keypress, and keyup and set various non-standardized properties on them). And that won't even work in a standard text input as isTrusted is set to False on events you generate. Simulating something like a Tab key press will require you writing code to try to replicate your browser's logic in determining what the next element should be.
Why would this be preferable over something like Selenium?
Cool idea - Web Extensions API are very powerful.
How would you go about installing the extension in a CI/CD setup? Can it be installed on headless Chrome?