Section 4.1.3: Characters and case
“In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-zA-Z0-9] and ISO 10646 (Unicode) characters U+00A1 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, or a hyphen followed by a digit. Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier “B&W?” may be written as “B\&W\?” or “B\26 W\3F”.”

As the “@” symbol is U+0040 (which is < U+00A1), I would venture that it would NOT validate.

Interesting review though. You raised some good points.