Why does child selector create different result when child is only element in class?

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Summary

The child selector in CSS can produce different results when applied to an element that is the only child of its parent, even if the styles seem identical. This discrepancy arises from how CSS handles inheritance and specificity. In the given example, applying font-size directly to the .header class and applying it to .header > h1 yield different font sizes for the h1 element.

Root Cause

The root cause of this discrepancy lies in how CSS applies styles:

  • When font-size is applied directly to the .header class, it sets the font size for the element and all its children, unless overridden.
  • When font-size is applied to .header > h1, it specifically targets the h1 element that is a direct child of .header, potentially overriding any inherited font size.

Why This Happens in Real Systems

This happens in real systems due to the following reasons:

  • CSS Inheritance: Properties like font-size are inherited by child elements from their parents, unless explicitly set.
  • CSS Specificity: The specificity of a selector determines which style rules take precedence. Directly targeting an element (like .header > h1) has higher specificity than targeting its parent (like .header).

Real-World Impact

The real-world impact includes:

  • Unpredictable styling if not understood properly
  • Difficulty in maintaining and debugging CSS code
  • Potential for cross-browser compatibility issues due to differences in how browsers handle inheritance and specificity

Example or Code

.header {
  grid-area: header;
  background-color: purple;
  text-align: center;
  color: #fffff5;
  font-size: 40px;
}

.header > h1 {
  font-size: 40px;
}

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Understanding and leveraging CSS inheritance and specificity rules
  • Using the browser’s developer tools to inspect and debug CSS styles
  • Applying consistent naming conventions and modular CSS practices to avoid conflicts and make debugging easier

Why Juniors Miss It

Juniors might miss this because:

  • Lack of understanding of CSS inheritance and specificity rules
  • Insufficient experience with debugging CSS issues
  • Not following best practices for writing and organizing CSS code