The invalid program for(;;){ a: continue a; } is parsed successfully. See ES5 section 12.7, specifically
A program is considered syntactically incorrect if [...] The program contains a continue statement with the optional Identifier, where Identifier does not appear in the label set of an enclosing (but not crossing function boundaries) IterationStatement.
as opposed to section 12.8 (break) which states
A program is considered syntactically incorrect if [...] The program contains a break statement with the optional Identifier, where Identifier does not appear in the label set of an enclosing (but not crossing function boundaries) Statement."
emphasis mine.