I have several apps from the app store, and none have the problem.
Also note that Word can open the files within the web page's folder. I can open that folder, and then open a .txt or .js or .css file in Word.
Word doesn't have a problem with opening the file, when I open it directly.
Word is failing when I open the html file that then is linking to the files in the folder.
Wed pages are a collection of elements, and all those elements are saved in a folder. The main html page is linking to the files in that folder.
Word only fails when the html file is linking to those files. Yet I can open them directly.
If this is a sandbox problem, then it's still a bug on Word because it can open the file, unless I do it the way outlined above.
Also, I'm reading the docs about sandbox and it explains Microsoft needs to describe their desired access control.
So it seems pretty clear to me, Microsoft has a bug. They are not requesting the proper access control.
https://developer.apple.com/library/content/documentation/Security/Conceptual/AppSandboxDesignGuide/AboutAppSandbox/AboutAppSandbox.html#//apple\_ref/doc/uid/TP40011183-CH1-SW1