I assume you are running straight from double clicking the html file from UE4 output (This will not work unless you are running it on firefox with uncompressed assets).
1.Firstly use only chrome/firefox to open the file (AFAIK only work on this two browser ATM).
2. Run HTML5LaunchHelper.exe in the folder(leave it running, this act as a “Server”)
3. You should be able to see “Starting Server at http://localhost:8000/” in the HTML5LaunchHelper command prompt(change the number of the localhost in step 4 if yours happen to be different)
4. In browser type in http://localhost:8000/
5. Click on your html file to run.
Hope it helps!