In each set of rules below, both rules apply to the same element and have selectors with identical specificity. IE before version 8 fails to apply the latter background value.
/* Tests 1 and 2: specificity (1,0,1) */
#foo div { background:red }
div #bar { background:green }
div #bar2 { background:red }
#foo2 div { background:green }
/* Tests 3 and 4: specificity (0,1,1) */
.foo div { background:red }
div .bar { background:green }
div .bar2 { background:red }
.foo2 div { background:green }
Particularly, the affected browsers seem to apply greater specificity when the id/class part of the selector comes before the element name.
Note: The "Browser Mode: IE7" emulation in IE8 does not display this bug, which severely reduces the value of this tool for testing IE7 rendering.