Content does not restrict its view and operation to a single display orientation, such as portrait or landscape, unless a specific display orientation is essential. (Level AA)
Examples where a particular display orientation may be essential are a bank check, a piano application, slides for a projector or television, or virtual reality content where binary display orientation is not applicable.
Description
Most of the handheld or tablet devices has the option to lock the orientation of the device. For instance, on an iPhone you can lock the orientation from the control Center (Control Center > lock rotation). Some users lock the orientation of their device for their comfort or some users, since the device is placed on a fixed place the orientation cannot be changed i.e. the devices fixed on the arm of a wheelchair.
1.3.4 orientation wants authors not to create any content or functionality that is restricted to one orientation. This will allow the users to view and operate the content of websites and applications in the orientation they are comfortable with or the default orientation of the device.
So, any application should respect the device specific orientation if the device orientation is locked, If the device orientation is not locked, the application or website should pick the default system level orientation. User agents pick the system level orientation either by the default device orientation or the orientation determined by the device sensors. In any of these conditions the content and functionality should remain unchanged.
Also keep in mind that the operating mechanisms can be different in different orientations. For example in landscape orientation the navigation menu may be a hide/ show mechanism while in portrait the same may be a ham burger menu.
Content or functionality changed by the size of the display of the device are not covered by this success criteria. In other words, 1.3.4 orientation does not talk about the changes in the content and functionality displayed in different screen resolutions.
Exceptions
Some situations restrict the benefit of viewing the content in both orientations. They are considered as exceptions of 1.3.4 orientation and are mentioned below.
- To capture a check, the  process in a banking app can be performed only in landscape due to the larger width compared with the height of the check. In landscape mode the width is usually more than height.
- A Piano app can only be accessed in landscape so that all the keys of the app can be effectively used.
- Slides viewed on projector or television cannot be rotated because of their fixed position.