The term screen orientation refers to whether a browser viewport is in landscape mode (that is, the width of the viewport is greater than its height), or else in portrait mode (the height of the viewport is greater than its width)

CSS provides the orientation media feature to allow adjusting layout based on screen orientation.

The Screen Orientation API provides a programmatic JavaScript API for working with screen orientation — including the ability to lock the viewport to a specific orientation.

One of the most common cases for orientation changes is when you want to revise the layout of your content based on the orientation of the device. For example, perhaps you want a button bar to stretch along the longest dimension of the device's display. By using a media query, you can do this easily and automatically.

Let's have an example with the following HTML code

<ul id="toolbar">
  <li>A</li>
  <li>B</li>
  <li>C</li>
</ul>

<p>
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis lacinia nisi nec
  sem viverra vitae fringilla nulla ultricies. In ac est dolor, quis tincidunt
  leo. Cras commodo quam non tortor consectetur eget rutrum dolor ultricies. Ut
  interdum tristique dapibus. Nullam quis malesuada est.
</p>

CSS relies on the orientation media query to handle specific styles based on the screen orientation

/* First let's define some common styles */

html,
body {
  width: 100%;
  height: 100%;
}

body {
  border: 1px solid black;

  -moz-box-sizing: border-box;
  box-sizing: border-box;
}

p {
  font: 1em sans-serif;
  margin: 0;
  padding: 0.5em;
}

ul {
  list-style: none;

  font: 1em monospace;
  margin: 0;
  padding: 0.5em;

  -moz-box-sizing: border-box;
  box-sizing: border-box;

  background: black;
}

li {
  display: inline-block;
  margin: 0;
  padding: 0.5em;
  background: white;
}

Once we have some common styles we can start defining a special case for the orientation

/* For portrait, we want the toolbar on top */

@media screen and (orientation: portrait) {
  #toolbar {
    width: 100%;
  }
}

/* For landscape, we want the toolbar stick on the left */

@media screen and (orientation: landscape) {
  #toolbar {
    position: fixed;
    width: 2.65em;
    height: 100%;
  }

  p {
    margin-left: 2em;
  }

  li + li {
    margin-top: 0.5em;
  }
}

And here's the result

Some devices (mainly mobile devices) can dynamically change the orientation of the screen based on their own orientation, ensuring that the user will always be able to read what's on the screen. While this behavior is perfectly suited for text content, there is some content that can be negatively affected by such a change. For example, games based on the orientation of the device could be messed up by such a change of the orientation.

The Screen Orientation API is made to prevent or handle such a change.

The orientationchange event is triggered each time the device change the orientation of the screen and the orientation itself can be read with the Screen.orientation property.

screen.addEventListener("orientationchange", () => {
  console.log(`The orientation of the screen is: ${screen.orientation}`);
});

Any web application can lock the screen to suits its own needs. The screen is locked using the screen.orientation.lock() method and unlocked using the screen.orientation.unlock() method.

The screen.orientation.lock() method accepts one of the following values to define the kind of lock to apply: any, natural. portrait-primary, portrait-secondary, landscape-primary, landscape-secondary, portrait, and landscape:

screen.orientation.lock();

It returns a promise that resolves after the lock succeeds.

• screen.orientation
• ScreenOrientation
• orientationchange event
• The orientation media query