Animation trigger test

This is an extremely experimental implementation of animation triggers proposed in #8942.

Scroll down the page to observe each type of trigger

Note there is empty space intentionally here

Once trigger

The following element animation is only triggered once.

Triggered once!

The following element animation is triggered every time .

Triggered every time!

The following element animation is only triggered every time, but reverses when leaving the range

Triggered every time, reverses on leaving range!