It turns out in CSS 3D, the order in which you specify for example
the rotateX(), rotateY() and rotateZ() transformations matter.
Each rotation is relative to the objects then-current position.
Impress.js being hardwired to always do rotateX->rotateY->rotateZ
was therefore limiting, and in fact there are some positions that
can never be reached with an xyz order. The new data-rotate-order=""
attribute allows to specify the order as a permutation of the 3
letters x, y, z, thus relaxing this limitation.
See http://openlife.cc/blogs/2016/october/3d-rotations-css-and-impressjs
for (much) more details.
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