.foo {
}

Programmatically, is:

if (element has a class name of "foo") {
}

Descendent selectors are && logic and commas are ||. It just gets more complicated from there, with things like combinators and pseudo selectors. Just look at all the ways styles can cascade.

Jeremy Keith:

If you find you can’t select something in the CSS, that’s a sign that you probably need to add another class name to the HTML. The complexity is confined to the markup in order to keep the CSS more straightforward, modular, and maintainable.

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