“iOS itself doesn’t support the HTML5 fullscreen API, so the only way to get a fullscreen mode is the Safari build-in landscape fullscreen mode.
On iPhone 6(S), you can choose between two display resolutions, standard and zoom:
iPhone 6(S) need to be set to standard to enable the built-in fullscreen.
iPhone 6(S) Plus need to be set to zoom mode to enable the built-in fullscreen.
See how to : https://support.apple.com/en-us/HT203073″.