[css-text] letter-spacing should not apply to the end edge of a line/span? #1518

The spec defines so, but some tests http://output.jsbin.com/mobaxex/ indicates no impls do this today.

While what the spec says looks reasonable in many cases, this does not look web-compatible, and conformant behavior may look worse if authors have put workaround, or in some use cases where it's not desired. Should we make the new behavior opt-in?

Edge platform data usage indicates 44% and Chrome usage indicates 30%.