Orientation
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.
In plain language
A page or app shouldn't force the screen to stay in only one orientation (portrait or landscape) — it should work in both, unless there's a real reason it can only work one way.
Why it matters
Some people have their device mounted in a fixed position — for example, on a wheelchair — and can't physically rotate it. If a site only works in one orientation, they can't use it at all.
In practice
Fails this criterion
A site locks itself to landscape mode only, and shows a message telling portrait users to rotate their device before they can continue.
Meets this criterion
The same site automatically adjusts its layout to work well in both portrait and landscape, without ever blocking or forcing a rotation.
Test it yourself
Open the site on a mobile device (or a device emulator) and try it in both portrait and landscape orientation. If it only works in one, or forces you to rotate, this criterion is failing.