The test is trying to determine whether a browser uses information in the HTTP header to determine the character encoding of a page (as described in the spec). 

To check it, we set a character encoding in the HTTP header and ensure that (1) there are no other character encoding declarations present, (2) that it is possible to distinguish the set encoding from any default encoding applied by the browser.

The question is then, how to know whether the encoding specified has actually been applied by the browser.  To do this I use a sequence of *bytes* in a class name that are interpreted as three different but legal *character* sequences in UTF-8, ISO 8859-1 and ISO 8859-15. The CSS file selector will only match the one of those character sequences that corresponds to the character encoding we are checking for, ie. it will only match if the HTTP header information has been recognised by the browser and the encoding of the document set to that encoding.

Clearer?

So, to repeat, we are checking that the browser picks up information from the HTTP header and uses it to set the character encoding of a page (as specified in the HTML5 spec).