h5bp/html5-boilerplate — comment on commit 0bf9143cd135df0b0357d4c7c50dcdb91dd85724
File: 404.html line 30
By sindresorhus:

That's subjective...

Like I said, you might prefer to use a class, but the right thing in this case is to use an ID.
#ID == unique
.class == reusable

`.container` is not reusable and only targets an unique element, and should therefor use an ID.

Not trying to start a CSS war here, I just want it to be as correct as it can be, so not to confuse beginners.

From the [spec](http://www.w3.org/TR/CSS2/selector.html#id-selectors):
The ID attribute of a document language allows authors to assign an identifier to one element instance in the document tree.