responsive-images

name	responsive-images
description	Implement performant responsive images with srcset, sizes, lazy loading, and modern formats (WebP, AVIF). Covers aspect-ratio for CLS prevention, picture element for art direction, and fetchpriority for LCP optimization. Use when: adding images to pages, optimizing Core Web Vitals, preventing layout shift, implementing art direction, or converting to modern formats.
license	MIT
metadata	{"keywords":["responsive images","srcset","sizes","lazy loading","webp","avif","aspect-ratio","CLS","LCP","fetchpriority"],"version":"1.0.0","last_updated":"2026-01-14T00:00:00.000Z"}

Responsive Images

Status: Production Ready ✅ Last Updated: 2026-01-14 Standards: Web Performance Best Practices, Core Web Vitals

---

Quick Start

Basic Responsive Image

<img
  src="/images/hero-800.jpg"
  srcset="
    /images/hero-400.jpg 400w,
    /images/hero-800.jpg 800w,
    /images/hero-1200.jpg 1200w,
    /images/hero-1600.jpg 1600w
  "
  sizes="(max-width: 640px) 100vw,
         (max-width: 1024px) 90vw,
         1200px"
  alt="Hero image description"
  width="1200"
  height="675"
  loading="lazy"
/>

Hero Image (LCP)

<img
  src="/images/hero-1200.jpg"
  srcset="
    /images/hero-800.jpg 800w,
    /images/hero-1200.jpg 1200w,
    /images/hero-1600.jpg 1600w
  "
  sizes="100vw"
  alt="Hero image"
  width="1600"
  height="900"
  loading="eager"
  fetchpriority="high"
/>

---

Configuration

Recommended Image Sizes

Use Case	Widths to Generate	Sizes Attribute
Full-width hero	800w, 1200w, 1600w, 2400w	100vw
Content width	400w, 800w, 1200w	(max-width: 768px) 100vw, 800px
Grid cards (3-col)	300w, 600w, 900w	(max-width: 768px) 100vw, 33vw
Sidebar thumbnail	150w, 300w	150px

Lazy Loading Rules

Image Position	loading	fetchpriority	Why
Hero/LCP	eager	high	Optimize LCP, prioritize download
Above fold (not LCP)	eager	omit	Load normally
Below fold	lazy	omit	Defer until near viewport
Off-screen carousel	lazy	omit	Defer until interaction

---

Common Patterns

Full-Width Responsive Image

<img
  src="/images/banner-1200.jpg"
  srcset="
    /images/banner-600.jpg 600w,
    /images/banner-1200.jpg 1200w,
    /images/banner-1800.jpg 1800w,
    /images/banner-2400.jpg 2400w
  "
  sizes="100vw"
  alt="Full width banner"
  width="2400"
  height="800"
  loading="lazy"
  class="w-full h-auto"
/>

Grid Card Image (3 columns)

<img
  src="/images/card-600.jpg"
  srcset="
    /images/card-300.jpg 300w,
    /images/card-600.jpg 600w,
    /images/card-900.jpg 900w
  "
  sizes="(max-width: 768px) 100vw,
         (max-width: 1024px) 50vw,
         33vw"
  alt="Card image"
  width="900"
  height="600"
  loading="lazy"
  class="w-full h-auto"
/>

Fixed Aspect Ratio Container

<div class="aspect-[16/9] overflow-hidden">
  <img
    src="/images/video-thumbnail-800.jpg"
    srcset="
      /images/video-thumbnail-400.jpg 400w,
      /images/video-thumbnail-800.jpg 800w,
      /images/video-thumbnail-1200.jpg 1200w
    "
    sizes="(max-width: 768px) 100vw, 800px"
    alt="Video thumbnail"
    width="800"
    height="450"
    loading="lazy"
    class="w-full h-full object-cover"
  />
</div>

Modern Formats (WebP + AVIF)

<picture>
  <source
    srcset="
      /images/hero-800.avif 800w,
      /images/hero-1200.avif 1200w,
      /images/hero-1600.avif 1600w
    "
    sizes="100vw"
    type="image/avif"
  />
  <source
    srcset="
      /images/hero-800.webp 800w,
      /images/hero-1200.webp 1200w,
      /images/hero-1600.webp 1600w
    "
    sizes="100vw"
    type="image/webp"
  />
  <img
    src="/images/hero-1200.jpg"
    srcset="
      /images/hero-800.jpg 800w,
      /images/hero-1200.jpg 1200w,
      /images/hero-1600.jpg 1600w
    "
    sizes="100vw"
    alt="Hero image"
    width="1600"
    height="900"
    loading="eager"
    fetchpriority="high"
  />
</picture>

Art Direction (Different Crops)

<picture>
  <source
    media="(max-width: 640px)"
    srcset="
      /images/product-portrait-400.jpg 400w,
      /images/product-portrait-800.jpg 800w
    "
    sizes="100vw"
  />
  <source
    media="(min-width: 641px)"
    srcset="
      /images/product-landscape-800.jpg 800w,
      /images/product-landscape-1200.jpg 1200w,
      /images/product-landscape-1600.jpg 1600w
    "
    sizes="(max-width: 1024px) 90vw, 1200px"
  />
  <img
    src="/images/product-landscape-1200.jpg"
    alt="Product image"
    width="1200"
    height="675"
    loading="lazy"
  />
</picture>

---

Error Prevention

Always Include Width and Height

Problem: Layout shift when images load (poor CLS score)

<!-- ❌ WRONG - causes layout shift -->
<img src="/image.jpg" alt="Image" loading="lazy" />

<!-- ✅ CORRECT - browser reserves space -->
<img
  src="/image.jpg"
  alt="Image"
  width="800"
  height="600"
  loading="lazy"
/>

Source: Web.dev - Optimize CLS

Don't Lazy Load LCP Images

Problem: Delayed LCP, poor Core Web Vitals score

<!-- ❌ WRONG - delays LCP -->
<img
  src="/hero.jpg"
  alt="Hero"
  loading="lazy"
/>

<!-- ✅ CORRECT - prioritizes LCP -->
<img
  src="/hero.jpg"
  alt="Hero"
  loading="eager"
  fetchpriority="high"
/>

Source: Web.dev - Optimize LCP

Use Width Descriptors (w), Not Density (x)

Problem: Browser can't choose optimal image for viewport

<!-- ❌ WRONG - only considers DPR -->
<img
  src="/image.jpg"
  srcset="/image.jpg 1x, /[e-mail] 2x"
  alt="Image"
/>

<!-- ✅ CORRECT - considers viewport + DPR -->
<img
  src="/image-800.jpg"
  srcset="
    /image-400.jpg 400w,
    /image-800.jpg 800w,
    /image-1200.jpg 1200w
  "
  sizes="(max-width: 768px) 100vw, 800px"
  alt="Image"
  width="800"
  height="600"
/>

Exception: Density descriptors are appropriate for fixed-size images like logos.

Always Include Alt Text

Problem: Fails accessibility, SEO, and screen readers

<!-- ❌ WRONG -->
<img src="/product.jpg" />

<!-- ✅ CORRECT - descriptive alt text -->
<img src="/product.jpg" alt="Red leather messenger bag with brass buckles" />

<!-- ✅ CORRECT - decorative images use empty alt -->
<img src="/decorative-line.svg" alt="" role="presentation" />

Aspect Ratio with object-fit

Problem: Image stretches or squashes when container size differs from image dimensions

<!-- ❌ WRONG - image distorts -->
<div class="w-full h-64">
  <img src="/image.jpg" alt="Image" class="w-full h-full" />
</div>

<!-- ✅ CORRECT - maintains aspect ratio -->
<div class="w-full h-64">
  <img
    src="/image.jpg"
    alt="Image"
    class="w-full h-full object-cover"
  />
</div>

---

Quick Reference

Sizes Attribute Patterns

<!-- Full width -->
sizes="100vw"

<!-- Content width (max 800px) -->
sizes="(max-width: 768px) 100vw, 800px"

<!-- Sidebar (fixed 300px) -->
sizes="300px"

<!-- 2-column grid -->
sizes="(max-width: 768px) 100vw, 50vw"

<!-- 3-column grid -->
sizes="(max-width: 768px) 100vw, (max-width: 1024px) 50vw, 33vw"

<!-- Responsive with max-width -->
sizes="(max-width: 640px) 100vw, (max-width: 1024px) 90vw, 1200px"

Common Aspect Ratios

Ratio	CSS	Use Case
16:9	aspect-[16/9]	Video thumbnails, hero images
4:3	aspect-[4/3]	Standard photos
3:2	aspect-[3/2]	DSLR photos
1:1	aspect-square	Profile pictures, Instagram-style
21:9	aspect-[21/9]	Ultrawide banners

object-fit Values

Value	Behavior	Use Case
cover	Fill container, crop edges	Card images, backgrounds
contain	Fit inside, preserve all content	Logos, product photos
fill	Stretch to fill	Avoid unless necessary
scale-down	Smaller of contain or original size	Mixed content sizes

Format Comparison

Format	Quality	File Size	Browser Support	Use Case
JPEG	Good	Medium	100%	Photos, complex images
PNG	Lossless	Large	100%	Logos, transparency
WebP	Excellent	Small	97%+	Modern browsers, photos
AVIF	Excellent	Smallest	90%+	Cutting-edge, fallback required

Recommended Strategy: AVIF → WebP → JPEG fallback using <picture>

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Resources

• Web.dev: Responsive Images
• MDN: Responsive Images
• Web.dev: Serve Images in Modern Formats
• Web.dev: Optimize Cumulative Layout Shift
• Cloudflare Images Documentation

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Token Efficiency: ~70% savings by preventing trial-and-error with srcset/sizes syntax Errors Prevented: 6 common responsive image issues documented above