This 1st sentence should have a green background.

This 2nd sentence should have a green background.

References:

CSS 2.1, Section 4.1.3 Characters and case (identifiers) says:
"in CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-z0-9] and ISO 10646 characters U+00A1 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, or a hyphen followed by a digit."

CSS 2.1, Section 4.1.1 tokenization CSS 2.1, G.2 Lexical scanner both gives
ident -?{nmstart}{nmchar}*
nmstart [_a-z]|{nonascii}|{escape}

Valid HTML 4.01! CSS compliant