Question: "Simple" question that I can't find the answer to -- What does `-webkit-perspective` actually do mathematically? (I know the effect it has, it basically acts like a focal-length control) e.g. what does `-webkit-perspective: 500` mean?!?

I need to find the on-screen location of something that's been moved using, among other things, `-webkit-perspective`

Answer: 
Check out <http://en.wikipedia.org/wiki/Perspective_projection#Diagram> 

After reading the previous comments and doing some research and testing I'm pretty sure this is correct. 

Notice that this is same for the Y coord too. 

Transformed X = Original X \* ( Perspective / ( Perspective - Z translation ) ) 

eg. 
Div is 500px wide 
Perspective is 10000px 
Transform is -5000px in Z direction

Transformed Width = 500 \* ( 10000 / ( 10000 - ( -5000 ) ) 
Transformed Width = 500 \* ( 10000 / 15000) = 500 \* (2/3) = 333px
@Domenic Oddly enough, the description "The matrix is computed by starting with an identity matrix and replacing the value at row 3, column 4 with the value -1/depth." has already been removed from the [The CSS 3D Transforms Module](http://www.w3.org/TR/css3-3d-transforms/#transform-functions) working draft now. Perhaps there might have been some inaccuracies with this description.

Well, as to the question what does the number in **perspective(<number>)** means, I think it could be seen as the distance between the position of the imagined camera and your computer screen.