Hello, it depends mostly on your OS. I've had some reports of this happening, for example, with MacOs or some browsers on linux. Try another browser, or, if it's still an issue, you should be able to run it as a local server. The open source text editor Brackets allows you to run files in a server very easily, by pressing a button. Another option is tu run the python simpleHttpServer from a terminal.
Let me know if it works for you.
Cheers!