In `w3` package the file `w3-display.el` create a *nil* face.

`(defvar w3-display-nil-face (w3-make-face nil "Stub face... don't ask."
t))`

This make `(facep nil)` not work properly, cause package `pos-tip` 's
function `pos-tip-show' unable to build the correct face when parameter
`tip-color' is *nil*.