Comment 3 for bug 800849

I suspect what you're seeing is an artefact of the line-layout problem
which has plagued WYSIWYG word processors for decades. Characters need
to be positioned using their real printed metrics rather than their
scaled metrics, to give an accurate impression of what the page will
look like.

This can lead to rounding errors which exhibit themselves as extra pixel
gaps between characters and uneven rhythm of characters.

As a test to confirm, could you try a line of l and a line of m
characters: lllllllllllllll and mmmmmmmmmmmmmmm.

Dave