Can CSS Transform a Block Element Inside an Inline Element into a Valid HTML Structure?

Can You Change a Block Element to Inline with CSS if it Contains a Block Element?

Despite the understanding that placing a block element within an inline element is improper, it becomes curious what happens when CSS is applied to change an inline element into a block element, which in turn contains another block element.

HTML Markup:

<div><p>This is a paragraph</p></div>

Applied CSS:

div {
  display: inline;
}

In this scenario, the inline element (DIV) contains a block element (P). The question arises, does this create an HTML validity issue?

HTML Validity and CSS Application

Opinions differ on how HTML validity is determined. Some argue that validity should be judged before CSS rules are applied, while others suggest post-CSS verification.

The CSS 2.1 Specification

The CSS 2.1 specification addresses this situation, stating that when an inline box contains an in-flow block-level box, the inline box breaks into two, one on each side of the block-level box. Moreover, anonymous block boxes are created to enclose the broken inline boxes and the block-level box.

Implementation and Consistency

While the CSS specification clearly outlines the behavior, its consistent implementation across browsers is uncertain. Some browsers may implement an alternative model, wrapping nested blocks in "anonymous line boxes" and drawing inline borders around them.

Conclusion

The question of validity in this context remains open to interpretation. While CSS rules specify the behavior of inline elements containing block elements, the implementation and consistency of this rule across browsers is not absolute. Designers are advised to use caution when employing such CSS styling to ensure desired results across multiple browsers.

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