interesting idea.

FWIW, tried with IE 6.x (changed IE element in the conditionals to "IE6") and Moz 1.7x. IE 6 honors the conditional comments; Moz renders the negative conditionals which return true.

Speculating now but Moz behavior may be because it demands w3c standard comments, < followed by two hyphens for open_comment and two hyphens > for close_comment.

(3.2.4) White space is not permitted between the markup declaration open delimiter("<!") and the comment open delimiter ("--"), but is permitted between the comment close delimiter ("--") and the markup declaration close delimiter (">"). A common error is to include a string of hyphens ("---") within a comment. Authors should avoid putting two or more adjacent hyphens inside comments.

from: http://www.w3.org/TR/html4/intro/sgmltut.html#idx-HTML

Would like to know more about which "IE oddities" lead to what thoughts about working around them. A link to a reference on the workings of the square bracket mechanism here would be nice to have, if permitted