Today I fought with a stylesheet for a good 15 minutes, wondering why a content element was overlapping a nav element, and why I couldn’t just give the nav element a higher z-index number. Turns out, z-index is ignored on elements that don’t have a position style attribute.
img {
z-index: 10000000; /* ignored */
}
img {
position: relative;
z-index: 10000000; /* works! */
}