Does anyone know any good "controllable" java HTML render?

By "controllable", what I want is:

1) I may not need to "physically" output the render result on screen,
render it in memory is all I need.
2) but I want to know each item's location after render!
For example, for this page, I want to know the relative (with respect
to the upper left (0,0)) location of the "My Groups", "Sign In", "Sign
Out", the location of the header of this post, etc.
In another words, I want to know:
Item/tag with value "My Groups" locates at pixel (600, 10) on the
screen after render.
Item/tag with value "New post" locates at pixel (700, 250) on the
screen after render. etc.

Any idea of it?

Thanks.