I would like to do the opposite of this tutorial which aligns HTML elements to ones in 3D space by projecting the 3D coordinates back onto the screen.

I am able to use unproject and some vector math to get pretty good results with location on the screen provided I'm using locations within the camera frustum. This no longer works when I want to use the position of HTML elements off-screen (i.e. places on the page where the user would need to scroll to) due to the perspective camera distortion.

How can I fix this?

Given a 2D coordinate (in particular, given an HTML element.offsetTop) and a perspective camera, how do I convert it into 3D space?

Thank you!