Using buttons with no icons or text

The Google Chrome "New tab" button is not labeled (see the top image). It contains no icon and no text (besides hovertext). There seems to be extensive discussion on using text vs. icon, but I can't seem to find anything on the precedent or utility of avoiding both icons and words for a label. This seems like it could get confusing quick, both for documents that refer to the button, and for ambiguity of such a button. It takes more words to describe steps to interact with an ambiguous button as opposed to a unique button. Should icon-free and text-free buttons be avoided?