responsive-images

SKILL.md

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, /image@2x.jpg 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>

Resources

Web.dev: Responsive Images

MDN: Responsive Images

Web.dev: Serve Images in Modern Formats

Web.dev: Optimize Cumulative Layout Shift

Cloudflare Images Documentation

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