Creating visible zero-width and zero-height glyphs


I'm playing with TTX/Fonttools and I'm trying to accomplish to get zero height and zero width chars:


All these six chars should be on the same spot (where the A is) and overlap eachother. This includes the chars on the second line.

I got the glyps to overlap horizontally by setting the width to 0 in the HMTX table.

Is there a way to set zero height as well? I read about the VMTX table but it looks like its only used for vertical fonts. Can you set zero height a different way? An line height of 0 for the entire font, instead of setting it for each character, would work as well.

And my zero width chars only appear when I prepend with a normal width character. Say I have A, B and C of 0 width, then


will be invisible, and