@-ms-viewport {width: device-width;}

1.
from: https://timkadlec.com/2012/10/ie10-snap-mode-and-responsive-design/

OCTOBER 17, 2012

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# In Windows 8, there are two “modes” of use: Metro mode and classic mode. Metro mode sports the spiffy new UI while classic is the same old boring Windows of yore.
# When you run Internet Explorer 10 in Metro mode (the default) there’s a cool new feature that lets you “snap” a window to the side so you can use two simultaneously. 
# when snapped, IE10 ignores the meta viewport tag for any viewport smaller than 400 pixels in width

# To get IE10 in snap mode to play nicely you have to use CSS Device Adaptation. 
# IE10 supports the @viewport rule with a -ms prefix.

# you need to start adding the @-ms-viewport rule to your CSS now to ensure your sites look as you would expect on Windows 8.

 

2 
About @viewport

from: https://developer.mozilla.org/en-US/docs/Web/CSS/@viewport

# The @viewport CSS at-rule lets you configure the viewport through which the document is viewed. It's primarily used for mobile devices, but is also used by desktop browsers that support features like "snap to edge" (such as Microsoft Edge).
# The use of <meta name="viewport"> tag overrides @viewport