I don’t know what it is. Same problem occurred when I tried to load JavaScript stored at internal storage. So to resolve this too, I had to create a html file with the JavaScript file location like (file:///storage/…), save it on the storage, finally display that html file. then it worked properly.
(I was using MathJax library of javascript)
I think that without having a html file saved at local storage, we couldn’t load any content from the storage devices through html codes.