2.1.3 [CSS-Level2-2009] Section 4.1.6, Blocks
V0180:
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A block starts with a left curly brace ({) and ends with the matching right curly brace (}). In between there may be any tokens, except that parentheses (( )), brackets ([ ]), and braces ({ }) must always occur in matching pairs and may be nested. Single (') and double quotes (") must also occur in matching pairs, and characters between them are parsed as a string. See Tokenization above for the definition of a string.
Quirks Mode and IE7 Mode (All Versions)
The following variations apply:
Nested blocks and strings are not parsed correctly.
Blocks and strings with terminating pairs that do not exist or are not matched are ignored.
Blocks and strings with matching terminating pairs of square brackets are parsed; the brackets do not affect the parsing of the rule.
IE8 Mode and IE9 Mode (All Versions)
Parsing is interrupted when a block, a curly brace, or a parenthesis is encountered before the body of the rule is completely defined after the first curly brace of the rule.