2025-08-01 — By Siddharth Jain · 7 min read

The Ultimate Guide to CSS Units, Positioning, and Display

Modern CSS offers you a range of ways to size, position, and lay out your webpage elements. This guide breaks down what every CSS beginner should know about units like px, em, rem, %, the differences between display: block and inline, and how position: absolute works.

Let’s dive in step-by-step!

📐 Understanding CSS Units: px, rem, em, %

1. px (Pixels)

• Definition: A pixel is a fixed, absolute unit. It represents a dot on the screen.
• Use-case: If you set font-size: 16px, it will always be 16 pixels on every screen.
• Pros: Consistent size, good for icons and sharp borders.
• Cons: Not responsive to user’s font scaling or device zoom.

.title {
  font-size: 24px;
  margin: 10px;
}

2. rem (Root EM)

• Definition: rem stands for "root em". It’s relative to the root (``) element’s font-size.
• Default: Most browsers set html { font-size: 16px } by default, so 1rem is 16px unless changed.
• Use-case: Great for setting base font sizes, paddings, and layouts that scale with user preferences.
• Pros: Easy to make your whole site scale by changing root font-size.
• Cons: All rem values rely on that single root size.

body {
  font-size: 1rem; /* 16px by default */
}
h1 {
  font-size: 2rem; /* 32px (2 x 16px) */
}

3. em (Element Relative)

• Definition: em is relative to the element’s own font size (or parent’s font size if not set).
• Nesting: ems compound; if a parent is 2em and child is 1.5em, child is 2 x 1.5 = 3em in actual size.
• Use-case: Useful for buttons, spacing, or elements that should scale with their container.
• Pros: Lets child components flexibly scale.
• Cons: Can become confusing with deep nesting due to compounding.

.button {
  font-size: 1em; /* Same as parent */
  padding: 0.5em 2em; /* Padding relative to font size */
}

4. % (Percentages)

• Definition: Percent values are relative to the parent element's dimension (width, height, font-size, etc.).
• Use-case: Fluid layouts, responsive designs, scalable containers.
• Example: width: 50% makes the width half of its parent’s width.

.container {
  width: 80%;
}
.child {
  width: 50%; /* 50% of .container's width */
}

• 5% specifically means 5 percent of the parent’s size.
  • If a parent is 1000px wide, width: 5% means 50px.

🧭 CSS Position: absolute;

What does position: absolute do?

• Definition: Removes the element from the normal flow and positions it relative to the nearest parent with a position set (relative, absolute, or fixed).
• Use-case: Creating overlays, dropdowns, tooltips, or placing items exactly.
• Properties: Use top, right, bottom, left to place the element.
• Important: If no parent is positioned, it will be relative to the `` or the initial viewport.

.parent {
  position: relative;
}
.child {
  position: absolute;
  top: 20px;
  left: 20px;
}

Visual Example:

• If you want a badge at the top-right of a button, absolutely position it inside a relatively-positioned button wrapper.

🧱 Display: block vs inline

1. display: block

• Definition: Element starts on a new line and takes the full width available.
• Examples: , , ,
• Behavior: Respects width, height, margin, and padding.
• Use-case: Layout containers, sections, cards.

.box {
  display: block;
  width: 70%;
  margin: 20px auto;
}

2. display: inline

• Definition: Element does not start on a new line; only takes as much width as its content.
• Examples: , , ``
• Behavior: Ignores width/height, only margin/padding left/right works.
• Use-case: Styling or highlighting short pieces of text inside a block.

.highlight {
  display: inline;
  color: orange;
}

3. Bonus: display: inline-block

• Combines benefits: behaves like inline (in text flow), but you can set width, height, margin, and padding.

👨💻 Practical Example

Absolutely Positioned! This is a highlighted word inside a block paragraph.

✅ Summary Table

📝 Final Tips

• Use rem for font sizes for accessibility.
• Use % for adaptable layouts.
• Use em for responsive spacing inside buttons/components.
• Use position: absolute for overlays, always within a relatively positioned parent.
• Use display: block for main content and inline for text, or inline-block for flexible components.