'clip' property doesn't work

for an image, if the position property isn't specified, 'clip' will have no effect on this image. why?

the following reference http://www.w3schools.com/CSS/pr_pos_clip.asp doesn't say that 'clip' works only if the position property is specified.

thx

http://www.w3.org/TR/CSS2/visufx.html#propdef-clip

"Applies to: absolutely positioned elements"

oh thx a lot