The current grammar for the contain-intrinsic-* properties is auto? [ none | <length> ], but WPT has tests that indicate a negative length is invalid, https://github.com/web-platform-tests/wpt/blob/7a9307ef9e1e88229ced4a29991270d20911669c/css/css-sizing/contain-intrinsic-size/parsing/contain-intrinsic-size-invalid.html#L22, which all browsers currently pass.

Should the grammar be updated to auto? [ none | <length [0,∞]> ]?

cc (@tabatkins, @fantasai, @jensimmons)