How to test html file for mobile responsiveness

Yes, simply open the .html file with Chrome or Firefox. These browsers have device mode.

You can change the screen size and see how it looks on mobile or tablet sized screens.

If you save your file .html you can open with firefox or chrome, but i suggest you to use firefox developer edition https://www.mozilla.org/it/firefox/developer/

If you use Firefox you can put in the menu -> development -> flexible display (ctrl+shift+m)

In Chrome (ctrl+maiusc+i + emulation)

In this page you can select the device to view.