Why Does Overlapping HTML Content Sometimes Not Obscure the Underlying Background?

What Causes the Overlapping Content Issue?

When two elements overlap in HTML/CSS, the content is not always covered by the background of the overlapping element. This occurs because of the specific painting order specified in the CSS rendering model.

CSS Painting Order

The CSS painting order determines the order in which elements are visually rendered on a web page. It is defined by a set of rules that specify which layers of content are painted first. Here's a simplified overview:

1. Background of root element: The background color of the root element () covers the entire canvas.
2. Backgrounds of child elements: If the child element is a block element (e.g.,
   ), its background color and image are painted on top of the root element's background, covering the entire width and height of the element.
3. Borders: The borders of the child element are painted on top of its background.
4. Text content: Finally, the text content of the child element is painted on top of the borders.

Issue in the Provided Example

In the given example, we have two elements:

1. .box: A
   with a red background.
2. .bottom: Another
   with a green background and negative margin applied, causing it to overlap .box.

According to the painting order:

1. The white background of is painted first, covering the entire page.
2. The red background of .box is painted next, and since it's a block element, it covers the entire width and height of the element.
3. The green background of .bottom is then painted, but due to the negative margin, it slides between the text content of .box and its background.
4. The text content of both elements is the last to be painted.

Conclusion

The observed behavior is a result of the predefined CSS painting order, which prioritizes text content rendering over background images or colors for overlapping elements.

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