@Egg, I agree that when using a slider for comparing one image to another, effects like bounce wouldn't be a good choice. Linear and fade are the only two viable options I can think of. The examples are well worth keeping, even though I haven't been brave enough to wander too far into animation other than the Xara offerings.

@Acorn, Your explanation as to why Website/Head is used makes perfect sense - notes are being made.
The clip-path mention was me poking things with a stick to see what the percentages did.

The links I'll look into and have a play. I have to be honest and say that I don't follow/understand a lot of your tutorials, and selfishly only play with those I see a use for. I therefore miss a lot of the rudiments.