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

Not trying to be difficult here, but isn't the whole point of H5BP to show best practices?
You might prefer to use classes, but the right thing here is to use an ID.
If the `.container` had contained generic properties that could be reused in different divs, it would be right, but in this case the properties are specific to only this div, and therefor an ID should be used.