this should be red... and it is, because browsers will generally allow this abuse. But a decent validator (CSS specific or html/css) will tell you something like this:

The "span" tag has no attributes. Attributes are normally used with the "span" element to provide functionality.

Normal text. red text and back to normal.

And this bold, green and with a grey background.

Black

Red? Not today! ...because span is NOT a class. However, turning again to our validator:

The "foobar" is not a recognized element in this selector. Is it misspelled?

Well, no, but while it works, there's no guarantee that it will tomorrow ... or even today, in some browsers.

dot_foobar (or even id foobar) would be a valid selector... used like this: text rendered as spec'ed by dot_foobar