When hints mode is active, a set of DOM nodes such as links, form fields, or images, will be tagged with numbers. These numbers are called hints. You can select one of the hinted items by typing its number and pressing enter.
Zero is a special number in hinting. Hint zero always refers to the top document in the buffer. Zero also has the property that it is never ambiguous (it is the only [reduced] number that begins with 0), so hinting exits immediately when you enter the number zero. This provides for some novel and very useful key sequences:
copy the current url
C-u f 0
open a new buffer with the current url
open the current document in an external program