Opacity flattening example does not show how transforms are flattened.

Referring to this:

https://github.com/GoogleChrome/samples/blob/5f07c2257876303575524e479b20fd48ddf877a1/css-opacity-force-flattening/index.html#L67-L75

That page should perhaps also show the undesirable and unfortunate effect that `opacity` flattening has on 3D transforms.