The GTK port misses support for CSS backdrop filters.
This was implemented in bug 138384 for Mac/iOS.
More info: https://webkit.org/blog/3632/introducing-backdrop-filters/
Status of support for the feature in other browsers: http://caniuse.com/#feat=css-backdrop-filter
On the surface it looks like the feature could be enabled by
adding the following in “PlatformGTK.cmake”:
WEBKIT_OPTION_DEFAULT_PORT_VALUE(ENABLE_FILTERS_LEVEL_2 PRIVATE ON)
but this alone is not enough for backdrop-filter to work. After
looking at how this was implemented in Cocoa, I think the crux of
the matter is that we have somehow to render normally the content
that falls behind the element to which backdrop filters are applied,
so then the result can be used to apply them. Cocoa seems to do
this using CoreAnimation, so with my (extremely) limited knowledge
of the graphics stack, we would need to implement the needed bits
in the texture mapper, right?