In programming, `f . g . h` is not the same as `h . f . g`.

In CSS, `<i class="a b c"></i>` is the same as `<i class="b c a"></i>`.

In programming, the order in which you define functions is not important — it's the order in which you use them which is important.

In CSS, it is the exact opposite.

To further drive the point home: assuming the names of these classes aren't lying, what colour is the text of this element?

<div class="text-red text-blue">foo</div>

It depends on the order in which the classes were defined. This also demonstrates why "pure" is a poor choice of word when describing anything to do with CSS. Nothing here is "pure".