First-child and last-child in CSS - Performance & Optimization

Performance: First-child and last-child

LOW IMPACT

This affects the browser's style calculation and layout performance when selecting elements based on their position in the DOM.

Styling only the first and last items in a list

CSS

li { color: black; }
li:first-child { color: red; }
li:last-child { color: blue; }

Only uses necessary positional selectors without redundant rules, minimizing style checks.

📈 Performance Gainsingle style calculation pass with minimal overhead

Styling only the first and last items in a list

CSS

li { color: black; }
li:first-child { color: red; }
li:last-child { color: blue; }
li:nth-child(n) { color: black; }

Using multiple complex positional selectors on many list items forces the browser to check each element's position repeatedly.

📉 Performance Costtriggers style recalculation for each list item, increasing with list size

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
:first-child and :last-child on small lists	Minimal DOM traversal	0	Low	[OK] Good
:first-child and :last-child on large lists repeatedly	High DOM traversal per element	0	Medium	[!] OK
Using JavaScript to add classes instead	Extra DOM manipulation	1	Low	[!] OK
Avoiding positional selectors entirely	Minimal DOM traversal	0	Low	[OK] Good

Rendering Pipeline

The browser matches :first-child and :last-child selectors during the Style Calculation stage by checking element positions in the DOM tree. This can cause extra work if many elements are involved.

→Style Calculation

→Layout

⚠️ BottleneckStyle Calculation due to positional checks

Optimization Tips

1Use :first-child and :last-child sparingly on large DOM trees.

2Combine positional selectors with classes to reduce complexity.

3Avoid redundant or overlapping positional selectors to minimize style recalculation.

Performance Quiz - 3 Questions

What is the main performance cost of using :first-child and :last-child selectors on a large list?

A Increases JavaScript execution time

B Triggers multiple reflows and repaints

C Extra style calculation time due to checking each element's position

D Blocks network requests

DevTools: Performance

How to check: Record a performance profile while loading or interacting with the page. Look for long Style Calculation times.

What to look for: High time spent in 'Recalculate Style' indicates costly selector matching like :first-child or :last-child on large DOMs.

Practice

1. What does the CSS selector :first-child do?

easy

A. It selects the last child element inside its parent.

B. It selects the first child element inside its parent.

C. It selects all child elements except the first.

D. It selects only the parent element.

Solution

1. Step 1: Understand the selector purpose

   The :first-child selector targets only the very first child element within a parent container.

2. Step 2: Compare with other options

   :last-child targets the last child, others do not match the description.

3. Final Answer:

   It selects the first child element inside its parent. -> Option B

4. Quick Check:

   :first-child = first child selected [OK]

Hint: First-child always picks the very first element inside a parent [OK]

Common Mistakes:

• Confusing :first-child with :last-child
• Thinking it selects all children
• Assuming it selects the parent

2. Which of the following is the correct CSS syntax to style the last child of a <ul> list?

easy

A. ul > li:last-child { color: red; }

B. ul:last-child { color: red; }

C. ul li:first-child { color: red; }

D. ul:last-child li { color: red; }

Solution

1. Step 1: Identify the target element

   The goal is to style the last <li> inside a <ul>, so the selector must target li elements that are last children.

2. Step 2: Check selector correctness

   ul > li:last-child correctly selects the last li directly inside ul. Other options either select the wrong element or use incorrect syntax.

3. Final Answer:

   ul > li:last-child { color: red; } -> Option A

4. Quick Check:

   Correct syntax for last child inside ul = ul > li:last-child { color: red; } [OK]

Hint: Use parent > child:last-child to style last child only [OK]

Common Mistakes:

• Using :last-child on the parent instead of the child
• Confusing :first-child with :last-child
• Missing the direct child combinator >

3. Given this HTML:

<ul>
  <li>Apple</li>
  <li>Banana</li>
  <li>Cherry</li>
</ul>

And CSS:

li:first-child { color: blue; }
li:last-child { color: green; }

What color will the text "Banana" have when rendered?

medium

A. Blue

B. Green

C. Black (default)

D. Both blue and green

Solution

1. Step 1: Identify which elements get styled

   li:first-child styles the first li (Apple), li:last-child styles the last li (Cherry).

2. Step 2: Determine Banana's position

   Banana is the second li, so it is neither first nor last child, so no color styles apply.

3. Final Answer:

   Black (default) -> Option C

4. Quick Check:

   Banana is middle child, no color applied [OK]

Hint: Only first or last child get styles, middle ones stay default [OK]

Common Mistakes:

• Assuming all list items get styled
• Confusing order of children
• Thinking styles cascade to siblings

4. This CSS code is intended to color the first and last paragraphs inside a <section> green and blue respectively:

section p:first-child { color: green; }
section p:last-child { color: blue; }

But only the first paragraph turns green, the last paragraph stays black. Why?

medium

A. Because the colors green and blue are invalid.

B. Because the CSS syntax is incorrect.

C. Because section cannot contain paragraphs.

D. Because p elements are not the first or last child of section.

Solution

1. Step 1: Understand :first-child and :last-child context

   These selectors check if the element is the very first or last child of its parent, regardless of type.

2. Step 2: Check if p is first or last child

   If other elements (like headings or divs) come before or after the p, then p is not first or last child, so styles won't apply.

3. Final Answer:

   Because p elements are not the first or last child of section. -> Option D

4. Quick Check:

   :first-child and :last-child depend on element position, not type [OK]

Hint: Check if element is truly first/last child, not just first/last of type [OK]

Common Mistakes:

• Assuming :first-child means first of type
• Ignoring other sibling elements
• Thinking syntax is wrong when it's position issue

5. You want to style the first and last <li> elements inside every <ul> differently, but only if the <li> is also the first or last child of its parent. Which CSS selectors correctly achieve this?

hard

A. ul > li:first-child { font-weight: bold; } and ul > li:last-child { font-style: italic; }

B. ul:first-child li { font-weight: bold; } and ul:last-child li { font-style: italic; }

C. ul li:first-child { font-weight: bold; } and ul li:last-child { font-style: italic; }

D. ul li:first-of-type { font-weight: bold; } and ul li:last-of-type { font-style: italic; }

Solution

1. Step 1: Understand the requirement

   We want to style only li elements that are the first or last child of their ul parent.

2. Step 2: Choose selectors that target direct children and correct position

   ul > li:first-child and ul > li:last-child select li elements that are direct children and first or last child respectively. This matches the requirement exactly.

3. Step 3: Evaluate other options

   ul li:first-child { font-weight: bold; } and ul li:last-child { font-style: italic; } misses the direct child combinator, which can cause incorrect matches if nested lists exist. ul:first-child li { font-weight: bold; } and ul:last-child li { font-style: italic; } incorrectly applies :first-child to ul. ul li:first-of-type { font-weight: bold; } and ul li:last-of-type { font-style: italic; } uses :first-of-type which selects first li regardless of position among siblings.

4. Final Answer:

   ul > li:first-child { font-weight: bold; } and ul > li:last-child { font-style: italic; } -> Option A

5. Quick Check:

   Direct child + :first-child and :last-child = ul > li:first-child { font-weight: bold; } and ul > li:last-child { font-style: italic; } [OK]

Hint: Use direct child combinator > with :first-child and :last-child [OK]

Common Mistakes:

• Omitting > combinator causing wrong matches
• Confusing :first-child with :first-of-type
• Applying selectors to wrong parent element