So what I need is simple: user presses something, user sees a shadow effect on new div (div centered window) on top of all page (with 1/4 size for example) alike 
Is it possible with some pure web-kit css art? Or javascript+images combination is needed? And how to do such thing?
From this quote:
and this one:
It seems as though you’re asking if it’s possible to create the “shadow” effect around the inner
divusing CSS.Chrome’s settings page is using CSS3’s
box-shadowto do this:box-shadowworks in these browsers: http://caniuse.com/css-boxshadowand the cross-browser CSS is:
http://jsfiddle.net/XHAbV/
If you need it to work in older versions of IE, you can use CSS3 PIE to emulate the
box-shadowin those browsers.If you’re after the JavaScript side of how to do this (a modal window), the other answer covers it quite thoroughly.