i did some more tests on upward drift. First, I copied the dropcap3 css from source into target & confirmed that that restored the original layout.
then I progressively lowered the drop cap font size from 4.6em to 4em, 3em, 2em...
eyeballing the opening word, at first I thought "optical illusion" but no.
with the opening word Given and a 3 or 4em G, the front gap of G aligns with the midpoint of i , but with a 2em G, the gap aligns with the dot above the i.
I've not resorted to holding a ruler against the PC screen but it does seem that the drop cap is being rendered higher, when its relative size is decreased - more of the top curve of the G is above an imaginary line drawn across the top of the following lower case characters???
or do I just need new glasses