Choosing the right unit in CSS depends on what you need to achieve with your design. Different units serve different purposes. Here are some points to help you decide which unit to use:

• Pixels (px): Fixed size for precise control.
  • Use when you need exact dimensions that do not change with screen size.
• Em and Rem: Scalable units for text and spacing.
  • Use when you want elements to scale with font size. Rem is based on the root font size, while em is based on the parent element's font size.
• Viewport Units (vw, vh): Responsive units based on the viewport size.
  • Use when you want elements to scale with the browser window. For example, set a section's width to 100vw to fill the screen width or use vh to set height relative to the viewport.

When to Use Each Unit

• Use Pixels (px) When:
  • You need strict control over the element size.
  • You are dealing with borders, icons, or other elements that require a fixed size.
• Use Em or Rem When:
  • You want the layout to adjust based on the user's font settings.
  • You need flexible text sizes that scale together with spacing.
  • Rem is ideal for global typography rules, while em is good for nested, local adjustments.
• Use VW and VH When:
  • You are designing elements that should fill the screen or a portion of it.
  • You want a layout or component to be responsive to the overall window size.
  • This is useful for hero sections, full-page layouts, or responsive typography that adapts with the viewport.

Practical Scenarios

• Fixed Navigation Bar:
  Use pixels if you want a precise height regardless of screen size.

• Body Text and Headers:
  Use rem to ensure consistent scaling, especially when users change the default browser font size.

• Full-Screen Backgrounds:
  Use vw and vh to make sure backgrounds or sections always cover the available screen area.