I can't get it to work. It might not even be possible.
It seems you need a <span> (inline elem) to get the background to closely match the text, but then you can't have a continuous image.
If there was some way to do masking between elements it would be easy. But the only masking stuff has to do with images, not text.
There is this CSS background-clip property which is close, but not quite. This is interesting: http://tympanus.net/Tutorials/ExperimentsBackgroundClipText/index2.html