I thought that was only in IE11, but apparently it has been around since IE8.
It is only available client-side though, and while you can feed it back to the server if relevant there (i.e. server-side rendering which most of the legacy code I'm responsible for does) you'd have to keep doing so in case the mode changes between requests.
Also you are forced to add in extra conditionals to your page whether you are working client- or server- side: you can respond to this property to deal with display differences via CSS selectors.
It is only available client-side though, and while you can feed it back to the server if relevant there (i.e. server-side rendering which most of the legacy code I'm responsible for does) you'd have to keep doing so in case the mode changes between requests.
Also you are forced to add in extra conditionals to your page whether you are working client- or server- side: you can respond to this property to deal with display differences via CSS selectors.