Is used to give the browser instructions on how to control the page's scaling and dimensions.
Attention: Is essential for the use of media-queries in css!
<meta name="viewport" content="width=device-width, initial-scale=1.0">
content may be:
width=device-width: Sets the pages width to the screen-size of the device
initial-scale=1.0: Sets the initial zoom level (in this case to 1)
user-scalable=no: Tells the browser if the user should be abled to scale a page or not.
Is not reccommended and marked as a warning in W3 Validator since zooming in an important accessibility feature, which is why it won't work in most browsers and is therefore marked as Unsupported.
Read more at W3Schools.
In the WHATWG WikiUp to date: v2.3.3