When a browser loads a web page it needs to apply styles, perform layout and then render the page so there’s something to look at. Filters kick in after all those steps and just before the page is copied to the screen. What they do is take a snapshot of the rendered page as a bitmap image, then perform some graphics magic on the pixels in the snapshot and then draw the result over the top of the original page image. One way to think of them is like a filter placed on the front of a camera lens. What you’re seeing through the lens is the outside world modified by the effect of the filter.
Everything you need to know about the upcoming CSS filters summarized by Alex Danilo.
As far as I can see, the only filter that interests me is ‚blur‘. And ‚drop-shadow‘ if it can detect transparent areas and follow the form of the element.