Why is width=device-width important for viewport tag?

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What does width=device-width do in the viewport meta tag?

A good viewport looks like this:

<meta name="viewport" content="width=device-width, initial-scale=1.0" />

This viewport meta tag tells the browser that the width of the page must be equal to the width of the device. Without width=device-width the browser determines what is likely the best size for the page. A responsive website however should work with small to large screen sizes.


Left an example of what happens when there is no or an invalid viewport tag, and on the right an example of a correct viewport meta tag

To prevent the browser from choosing the wrong width for the page, set the viewport meta tag with width=device-width.

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