I have the following problem: I am developing an HTML\CSS interface for a web application that have to be compatible with Internet Explorer 8.
What is the best way to test it on this old browser?
IE11 has a rough IE8 emulation mode known as Enterprise Mode (EMIE). This is available in any version of Windows supporting IE11, including Windows 10 alongside Microsoft Edge (which itself does not have any IE emulation options). On Windows 10 you won't find IE in the list of apps, but you can find it using search/Cortana.
But for the most authentic experience you'll want to use a virtual machine that has an actual copy of IE8 preinstalled.