Kerning allows precise alignment of text. 3rd party software uses kerning to control printing when exporting to SVG
We support horizontal kerning, but not vertical kerning. Most helpful would be if you could attach a test case! :)
My bad. The title should not include the word vertical. If you go to the posted URL it has several tests for horizontal kerning. The nightly WebKit builds do not pass any of these tests.
Fixed in trunk.