Although dangerous tricks such as behavior: url('some.htc') aren't available in modern browsers by default, if you are using the X-UA-Compatible header or meta tag to force Microsoft Internet Explorer in a different render mode, or if you are omitting the document type declaration, Internet Explorer will use the behavior of older versions where such things are available.
A demonstration (note: I can't get this to work on IE 11): http://jsbin.com/siyuhika/1/