Adding to my previous comment, you can even run chrome webpage without address bar on macos : /Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --app=https://google.com
Why expose api. One could run web server on localhost and thats it. Open it with any browser and enjoy your crosplatform app. I think https://blockstack.org is doing something similar.
The only way I think a common runtime could work is if Electron used the Runtime of the Browser. If every browser exposed an API to run JavaScript and Render HTMl using the browser's Engine that'd be great.