The following demo tests whether an illegal decimal value is parsed in the context of a z-index.
NOTE: Both SPAN elements in the example below are positioned absolutley relative to their containing block.
A red-colored SPAN element with an illegal z-index value of 99.9 precedes (in the markup order) a green-colored SPAN element which, since no z-index value is specified, establishes a local stacking context level of '0'. Since a decimal value is illegal in this context this decimal value should not be parsed, thus, the last element in the markup order (green-colored SPAN) should take precedence in the stacking order instead.