Smartphones usually do not come with a very large screen and when watching videos online, you want to make the maximum use of the screen in your smartphone so you tilt the phone sideways to rotate the screen and switch to the full-screen mode. But even if by tilting the phone to the side, if your phone is not rotating the screen then there are some settings that you must configure in Android to enable automatic rotating of the screen. Here is how:
First of all you have to make sure that the auto-rotate function is toggled on in your phone. For this, you have to use two fingers to drag down the notification bar to see the quick-settings panel and tap on the auto-rotate button to toggle this option on. If you tap on this auto-rotate button once again, then the phone switches to the portrait only mode which does not allow automatic rotation.
You can toggle the same options from the Android settings screen. You have to open Android settings, then open the Display settings and finally you can find the desired options under When device is rotated. You can choose either to stay in the portrait mode or rotate the contents of the screen when the device is rotated.
If these options are not present you cannot find them in your Android phone, then perhaps you have to toggle the auto-rotation using the accessibility settings. After opening the Android settings, open the Accessibility settings and then select the checkbox labeled Auto-rotate screen.
Using these various methods, you can make easily make Android rotate the screen when you rotate the device. The location of these options might vary a little depending on the ROM you are using, but they should be easy to find in any Android based phone.