Understanding the z-index Property in CSS

The 'z-index' property in CSS is often misunderstood or overlooked by many developers, yet it plays a crucial role in determining the stacking order of elements on a webpage. The question pointed out that the 'z-index' property in CSS does not control the opacity, zoom level, or horizontal alignment of an element, but rather the stacking order. But what does this mean?

Imagine having several papers stacked on a table. The paper that is on the top of the stack obscures the ones underneath it. Similarly, in CSS, when elements overlap, the z-index property helps determine which element is brought to the front, thus becoming visible.

The z-index property can accept either a positive or a negative integer. Higher values mean that the element will be closer to the top of the stack. Lower or negative values will move the element deeper in the stack. Elements without a defined z-index default to a stacking order of zero.

Here's a simple example:

div {
    position: absolute;
    left: 0px;
    top: 0px;
}

#div1 {
    z-index: 1;
    background: red;
}

#div2 {
    z-index: 2;
    background: blue;
}

In this case, #div2 will appear on top of #div1 because it has a higher z-index.

It's important to note that z-index only works on elements with a position property set to absolute, relative, or fixed.

Understanding how the z-index property works can improve your ability to control the visual hierarchy of your design. It's a powerful tool for creating more complex layouts, and mastering it is integral to harnessing the full potential of CSS.

When using z-index, remember that less is more. Try to keep your z-index values as low as possible. This way, it's easier for you and other developers to understand the stacking context of your website. Additionally, take note that setting absurdly high values for the sake of overcoming stacking issues could lead to less maintainable code down the line.

In conclusion, 'z-index' is a powerful CSS property that controls the stacking order of elements. Properly used, it ensures the correct element overlay order, improving both the appearance and user experience of a website.