run yarn && yarn webpack

open indexA.html in chrome, open devtools console, you'll see an error, because "buildin/global.js" is being included in vendor-scripts which is not loaded on indexA.html page, page B works well

you can try and add react into the vendor-scripts like this

"vendor-scripts": [
	"react",
	"mobx",
],

and now "process/browser.js" gone from a.js as well, it is tho required to load module "js-htmlencode" which is taken as example.