No reason to expect anything different. If you want an inner shadow, add a second one to the declaration that starts with the keyword inset.
E.g. -webkit-box-shadow: 0 0 40px 0 rgba(0,0,0,0.5), inset 0 0 40px 0 rgba(0,0,0,0.5);
.
Note that elements that are descendants of the element with the box shadow will cover the inner shadow.
Also note that some older versions of modern browsers only support one shadow declaration at a time, but I think that set of browsers/versions is quite small.