Media queries allow us to apply CSS styles depending on a device's general type (such as print vs. screen) or other characteristics such as screen resolution or browser viewport width.

Media queries can be used to check many things, such as:

1. width and height of the viewport
2. width and height of the device
3. orientation (is the tablet/phone in landscape or portrait mode?) resolution.

Using media queries are a popular technique for delivering a tailored style sheet to desktops, laptops, tablets, and mobile phones (such as iPhone and Android phones).

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Media queries are a key part of responsive web design, as they allow you to create different layouts depending on the size of the viewport, but they can also be used to detect other things about the environment your site is running on.