This is somewhat related to ISSUE 4 in the spec. We currently specify mix-blend-mode conditionally if we have a crossfade.

This, however, has unintended and sometimes surprising effects for developers trying to customize old/new animations. @argyleink had an example where the old content did a slide out to the left and new content did a slide in from the left: the overlap between the two contents did not "look right", since the expectation is to see a normal blend mode.

My suggestion is to consider putting mix-blend-mode to be a part of the UA animation instead, maybe something like

@keyframes -ua-mix-blend-mode-plus-lighter {
  from { mix-blend-mode: plus-lighter }
  to { mix-blend-mode: plus-lighter }
}

and adding that into the animation-name for ua generated css. As an aside, mix-blend-mode is not animatable, but would need to be at least discretely animatable for this to work.

It's a bit of a hack, but I worry that having mix-blend-mode that is added for pairs automatically may cause more developer pain.

Any thoughts?