Starts-with and ends-with selectors in CSS - Performance & Optimization

Performance: Starts-with and ends-with selectors

MEDIUM IMPACT

These selectors affect how quickly the browser matches elements to styles during rendering.

Styling elements based on attribute prefixes or suffixes

CSS

.btn-primary { color: blue; } .active { font-weight: bold; }

Using simple class selectors lets the browser quickly match styles using fast lookup.

📈 Performance Gainstyle matching is near-instant, reducing LCP and improving responsiveness

Styling elements based on attribute prefixes or suffixes

CSS

[class^='btn-'] { color: blue; } [class$='-active'] { font-weight: bold; }

These selectors require the browser to check attribute values character-by-character for many elements, causing slower style matching.

📉 Performance Costtriggers slower style calculation on large DOMs, increasing LCP by tens of milliseconds

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
[attr^='value'] or [attr$='value']	Many attribute checks per element	0 (style only)	Low to medium (style recalculation)	[!] OK
.class or #id selectors	Fast direct lookup	0	Low	[OK] Good

Rendering Pipeline

Starts-with and ends-with selectors are evaluated during the Style Calculation stage, where the browser matches CSS rules to DOM elements by checking attribute values.

→Style Calculation

⚠️ BottleneckStyle Calculation is slower because attribute substring matching is more complex than simple class or ID matching.

Core Web Vital Affected

LCP

These selectors affect how quickly the browser matches elements to styles during rendering.

Optimization Tips

1Avoid overusing starts-with and ends-with selectors on large DOMs.

2Prefer simple class or ID selectors for faster style matching.

3Use DevTools Performance panel to spot slow style recalculations.

Performance Quiz - 3 Questions

Why can starts-with ([attr^='value']) selectors slow down page rendering?

A Because they increase the number of DOM nodes

B Because they require checking attribute values character-by-character for many elements

C Because they cause layout shifts

D Because they block network requests

DevTools: Performance

How to check: Record a performance profile while loading the page. Look for long Style Calculation times in the summary.

What to look for: High time spent in 'Style Recalculation' indicates expensive selectors like starts-with or ends-with.

Practice

1. Which CSS selector targets all elements with an href attribute that starts with "https"?

easy

A. a[href*="https"]

B. a[href$="https"]

C. a[href^="https"]

D. a[href~="https"]

Solution

1. Step 1: Understand the starts-with selector syntax

   The starts-with selector uses the caret symbol (^) inside square brackets to match attribute values that begin with a specific string.

2. Step 2: Match the selector to the question

   The selector a[href^="https"] matches all anchor tags with href attributes starting with "https".

3. Final Answer:

   a[href^="https"] -> Option C

4. Quick Check:

   Starts-with selector = ^ = a[href^="https"] [OK]

Hint: Starts-with selector uses ^ inside attribute brackets [OK]

Common Mistakes:

• Confusing starts-with (^) with ends-with ($)
• Using *= which means contains, not starts-with
• Using ~ which matches whole words in space-separated values

2. Which of the following is the correct CSS syntax to select elements with a class attribute ending with "-btn"?

easy

A. [class$="-btn"]

B. .class^="-btn"

C. .class$="-btn"

D. [class^="-btn"]

Solution

1. Step 1: Identify the ends-with selector syntax

   The ends-with selector uses the dollar sign ($) inside square brackets to match attribute values that end with a specific string.

2. Step 2: Apply correct syntax for class attribute

   To select elements whose class attribute ends with "-btn", use [class$="-btn"]. The dot (.) is for class names, not attribute selectors.

3. Final Answer:

   [class$="-btn"] -> Option A

4. Quick Check:

   Ends-with selector = $ inside [] = [class$="-btn"] [OK]

Hint: Ends-with selector uses $ inside attribute brackets [OK]

Common Mistakes:

• Using dot (.) with attribute selectors incorrectly
• Confusing starts-with (^) with ends-with ($)
• Missing square brackets for attribute selectors

3. Given this HTML:

<a href="https://example.com">Link1</a>
<a href="http://example.com">Link2</a>
<a href="https://secure.com">Link3</a>

Which links will be styled by the CSS selector a[href^="https"] { color: red; }?

medium

A. All three links

B. Only Link1

C. Link2 and Link3

D. Link1 and Link3

Solution

1. Step 1: Identify which href values start with "https"

   Link1 has href="https://example.com" and Link3 has href="https://secure.com". Both start with "https". Link2 starts with "http" (no s).

2. Step 2: Apply the starts-with selector effect

   The selector a[href^="https"] styles only elements whose href attribute starts with "https". So Link1 and Link3 get styled.

3. Final Answer:

   Link1 and Link3 -> Option D

4. Quick Check:

   Starts-with "https" matches Link1 & Link3 [OK]

Hint: Starts-with selector matches only beginning of attribute value [OK]

Common Mistakes:

• Including links starting with "http" without s
• Confusing contains (*) with starts-with (^)
• Assuming all links get styled

4. You wrote this CSS but it doesn't style any elements:

input[name^="user"] { background-color: yellow; }

HTML:

<input name="login">
<input name="signup">
<input name="emailuser">

What is the likely problem?

medium

A. The selector should use $ instead of ^

B. The attribute values do not start with "user"

C. The input elements need IDs for this selector

D. The selector syntax is invalid

Solution

1. Step 1: Check attribute values against selector

   The selector targets inputs with name attributes starting with "user". The inputs have names "login", "signup", and "emailuser". None of these start with "user".

2. Step 2: Identify why no elements styled

   Since none of the name attributes start with "user", no elements match the selector, so nothing gets styled.

3. Final Answer:

   The attribute values do not start with "user" -> Option B

4. Quick Check:

   Starts-with selector matches only beginning of attribute value [OK]

Hint: Check if attribute values actually start with the given string [OK]

Common Mistakes:

• Assuming ends-with ($) is needed instead of starts-with (^)
• Thinking IDs are required for attribute selectors
• Believing selector syntax is wrong when it is correct

5. You want to style all img elements whose src attribute starts with "https://cdn." and ends with ".png". Which CSS selector correctly targets these images?

hard

A. img[src^="https://cdn."][src$=".png"]

B. img[src*="https://cdn."][src*=".png"]

C. img[src$="https://cdn."][src^=".png"]

D. img[src^=".png"][src$="https://cdn."]

Solution

1. Step 1: Understand combined attribute selectors

   You can combine multiple attribute selectors to match elements that satisfy all conditions. Here, one selector checks if src starts with "https://cdn." and another checks if src ends with ".png".

2. Step 2: Match correct starts-with and ends-with syntax

   Starts-with uses ^ and ends-with uses $. So the correct selector is img[src^="https://cdn."][src$=".png"].

3. Final Answer:

   img[src^="https://cdn."][src$=".png"] -> Option A

4. Quick Check:

   Combine ^ and $ selectors correctly [OK]

Hint: Combine ^ and $ selectors to match start and end [OK]

Common Mistakes:

• Using * (contains) instead of ^ or $
• Swapping ^ and $ symbols
• Incorrect order or syntax of attribute selectors