Responsive Images Skill

This skill covers modern responsive image techniques: resolution switching with srcset and sizes, art direction with <picture>, and modern format support with graceful fallbacks.

Philosophy

Images should:

1. Serve optimal files - Right size for the display context
2. Use modern formats - AVIF, WebP with JPEG/PNG fallback
3. Prioritize correctly - LCP images load first, others lazy load
4. Work everywhere - Fallbacks for older browsers

---

The Image Loading Attributes

Every image should have appropriate loading attributes:

<img src="photo.jpg"
     alt="Descriptive text"
     loading="lazy"
     decoding="async"
     fetchpriority="auto"/>

Attribute Reference

Attribute	Values	Purpose
loading	eager, lazy	When to load the image
decoding	sync, async, auto	How to decode the image
fetchpriority	high, low, auto	Network priority hint

Loading Strategy by Context

Image Type	loading	fetchpriority	decoding
Hero/LCP image	eager	high	async
Above fold	eager	auto	async
Below fold	lazy	auto	async
Thumbnails	lazy	low	async
Background decorative	lazy	low	async

---

Resolution Switching with srcset and sizes

The Problem

A single image file can't serve all contexts:

• A 2000px image wastes bandwidth on mobile
• A 400px image looks blurry on retina displays
• Screen size and pixel density vary widely

The Solution: srcset and sizes

<img src="photo-800.jpg"
     srcset="photo-400.jpg 400w,
             photo-800.jpg 800w,
             photo-1200.jpg 1200w,
             photo-1600.jpg 1600w"
     sizes="(max-width: 600px) 100vw,
            (max-width: 1200px) 50vw,
            800px"
     alt="Product photograph"
     loading="lazy"
     decoding="async"/>

How It Works

1. srcset lists available image files with their widths (w descriptor)
2. sizes tells the browser how wide the image will display
3. Browser calculates optimal file based on viewport + pixel density
4. src provides fallback for browsers without srcset support

The sizes Attribute

sizes uses media conditions to describe rendered width:

sizes="(max-width: 600px) 100vw,
       (max-width: 1200px) 50vw,
       800px"

This means:

• At 600px viewport or less: image displays at 100% viewport width
• At 601-1200px viewport: image displays at 50% viewport width
• Above 1200px: image displays at 800px fixed width

Common sizes Patterns

Context	sizes Value
Full-width hero	100vw
Content image (max 800px)	(max-width: 800px) 100vw, 800px
Two-column grid	(max-width: 600px) 100vw, 50vw
Three-column grid	(max-width: 600px) 100vw, (max-width: 900px) 50vw, 33vw
Card thumbnail	(max-width: 600px) 100vw, 300px
Gallery thumbnail	(max-width: 600px) 50vw, 220px

---

Art Direction with <picture>

When to Use <picture>

Use <picture> when you need different images for different contexts:

• Different crops (portrait mobile, landscape desktop)
• Different aspect ratios
• Showing/hiding details at different sizes
• Different content entirely

Basic Art Direction

<picture>
  <source media="(max-width: 600px)"
          srcset="photo-mobile.jpg"/>
  <source media="(max-width: 1200px)"
          srcset="photo-tablet.jpg"/>
  <img src="photo-desktop.jpg"
       alt="Product photograph"
       loading="lazy"
       decoding="async"/>
</picture>

Art Direction with srcset

Combine art direction with resolution switching:

<picture>
  <!-- Mobile: portrait crop -->
  <source media="(max-width: 600px)"
          srcset="photo-mobile-400.jpg 400w,
                  photo-mobile-600.jpg 600w"
          sizes="100vw"/>

  <!-- Tablet: square crop -->
  <source media="(max-width: 1200px)"
          srcset="photo-tablet-600.jpg 600w,
                  photo-tablet-900.jpg 900w"
          sizes="50vw"/>

  <!-- Desktop: landscape crop -->
  <img src="photo-desktop-800.jpg"
       srcset="photo-desktop-800.jpg 800w,
               photo-desktop-1200.jpg 1200w,
               photo-desktop-1600.jpg 1600w"
       sizes="800px"
       alt="Product photograph"
       loading="lazy"
       decoding="async"/>
</picture>

---

Modern Image Formats

Format Comparison

Format	Compression	Browser Support	Use Case
AVIF	Best (50% smaller than JPEG)	Chrome, Firefox, Safari 16+	Primary modern format
WebP	Very good (25-35% smaller)	All modern browsers	Fallback for AVIF
JPEG	Good	Universal	Final fallback
PNG	Lossless	Universal	Transparency, screenshots

Format Fallback Pattern

<picture>
  <source srcset="photo.avif" type="image/avif"/>
  <source srcset="photo.webp" type="image/webp"/>
  <img src="photo.jpg"
       alt="Descriptive text"
       loading="lazy"
       decoding="async"/>
</picture>

Complete Pattern: Formats + Resolution + Art Direction

<picture>
  <!-- Mobile: AVIF -->
  <source media="(max-width: 600px)"
          type="image/avif"
          srcset="photo-mobile-400.avif 400w,
                  photo-mobile-600.avif 600w"
          sizes="100vw"/>

  <!-- Mobile: WebP -->
  <source media="(max-width: 600px)"
          type="image/webp"
          srcset="photo-mobile-400.webp 400w,
                  photo-mobile-600.webp 600w"
          sizes="100vw"/>

  <!-- Mobile: JPEG -->
  <source media="(max-width: 600px)"
          srcset="photo-mobile-400.jpg 400w,
                  photo-mobile-600.jpg 600w"
          sizes="100vw"/>

  <!-- Desktop: AVIF -->
  <source type="image/avif"
          srcset="photo-800.avif 800w,
                  photo-1200.avif 1200w"
          sizes="(max-width: 1200px) 50vw, 800px"/>

  <!-- Desktop: WebP -->
  <source type="image/webp"
          srcset="photo-800.webp 800w,
                  photo-1200.webp 1200w"
          sizes="(max-width: 1200px) 50vw, 800px"/>

  <!-- Desktop: JPEG fallback -->
  <img src="photo-800.jpg"
       srcset="photo-800.jpg 800w,
               photo-1200.jpg 1200w"
       sizes="(max-width: 1200px) 50vw, 800px"
       alt="Descriptive text"
       loading="lazy"
       decoding="async"/>
</picture>

---

Common Image Patterns

Hero Image (LCP)

<picture>
  <source type="image/avif"
          srcset="hero-800.avif 800w,
                  hero-1200.avif 1200w,
                  hero-1920.avif 1920w"
          sizes="100vw"/>
  <source type="image/webp"
          srcset="hero-800.webp 800w,
                  hero-1200.webp 1200w,
                  hero-1920.webp 1920w"
          sizes="100vw"/>
  <img src="hero-1200.jpg"
       srcset="hero-800.jpg 800w,
               hero-1200.jpg 1200w,
               hero-1920.jpg 1920w"
       sizes="100vw"
       alt="Hero image description"
       loading="eager"
       fetchpriority="high"
       decoding="async"/>
</picture>

Content Image

<picture>
  <source type="image/avif"
          srcset="content-400.avif 400w,
                  content-800.avif 800w,
                  content-1200.avif 1200w"
          sizes="(max-width: 800px) 100vw, 800px"/>
  <source type="image/webp"
          srcset="content-400.webp 400w,
                  content-800.webp 800w,
                  content-1200.webp 1200w"
          sizes="(max-width: 800px) 100vw, 800px"/>
  <img src="content-800.jpg"
       srcset="content-400.jpg 400w,
               content-800.jpg 800w,
               content-1200.jpg 1200w"
       sizes="(max-width: 800px) 100vw, 800px"
       alt="Content image description"
       loading="lazy"
       decoding="async"/>
</picture>

Card Thumbnail

<picture>
  <source type="image/avif"
          srcset="thumb-300.avif 300w,
                  thumb-450.avif 450w"
          sizes="(max-width: 600px) 100vw, 300px"/>
  <source type="image/webp"
          srcset="thumb-300.webp 300w,
                  thumb-450.webp 450w"
          sizes="(max-width: 600px) 100vw, 300px"/>
  <img src="thumb-300.jpg"
       srcset="thumb-300.jpg 300w,
               thumb-450.jpg 450w"
       sizes="(max-width: 600px) 100vw, 300px"
       alt="Card thumbnail description"
       loading="lazy"
       decoding="async"/>
</picture>

Gallery Image

<picture>
  <source type="image/avif"
          srcset="gallery-220.avif 220w,
                  gallery-330.avif 330w,
                  gallery-440.avif 440w"
          sizes="(max-width: 600px) 50vw, 220px"/>
  <source type="image/webp"
          srcset="gallery-220.webp 220w,
                  gallery-330.webp 330w,
                  gallery-440.webp 440w"
          sizes="(max-width: 600px) 50vw, 220px"/>
  <img src="gallery-220.jpg"
       srcset="gallery-220.jpg 220w,
               gallery-330.jpg 330w,
               gallery-440.jpg 440w"
       sizes="(max-width: 600px) 50vw, 220px"
       alt="Gallery image description"
       loading="lazy"
       decoding="async"/>
</picture>

---

Simplified Patterns

When Full <picture> Is Overkill

For simpler cases, srcset alone may suffice:

<!-- Resolution switching only, no format variants -->
<img src="photo-800.jpg"
     srcset="photo-400.jpg 400w,
             photo-800.jpg 800w,
             photo-1200.jpg 1200w"
     sizes="(max-width: 600px) 100vw, 50vw"
     alt="Description"
     loading="lazy"
     decoding="async"/>

Minimum Viable Responsive Image

At minimum, always include:

<img src="photo.jpg"
     alt="Descriptive text"
     loading="lazy"
     decoding="async"
     width="800"
     height="600"/>

The width and height attributes prevent layout shift.

---

Image Sizing Strategy

Images in Container-Queried Components

The HTML sizes attribute uses viewport media queries, not container queries. For images inside container-queried components, the sizes may not accurately describe rendered width.

Strategies:

1. Generous sizes estimation - Provide srcset that covers the full range:

<!-- Inside a container-queried card that could be 300-800px wide -->
<img srcset="photo-300.jpg 300w,
             photo-450.jpg 450w,
             photo-600.jpg 600w,
             photo-800.jpg 800w"
     sizes="(max-width: 600px) 100vw, 800px"
     alt="Product photo"/>

2. CSS-controlled sizing - Let CSS and container queries control the rendered size:

@layer components {
  product-card {
    container-type: inline-size;
  }

  product-card img {
    width: 100%;
    height: auto;
    aspect-ratio: 4/3;
    object-fit: cover;
  }

  @container (min-width: 500px) {
    product-card img {
      width: 40%;
    }
  }
}

The browser still selects from srcset based on rendered size after CSS is applied.

3. Use object-fit with container units for fluid sizing:

product-card img {
  width: min(100%, 20cqi);
  aspect-ratio: 1;
  object-fit: cover;
}

Note: Future CSS may support container-based image selection, but for now, provide a robust srcset range.

Standard Breakpoints for srcset

Generate images at these widths to cover common scenarios:

Width	Use Case
400w	Mobile thumbnails
600w	Mobile full-width
800w	Tablet/small desktop
1200w	Desktop content
1600w	Large desktop/retina
1920w	Full-width heroes

Calculating sizes

1. Measure rendered width at key breakpoints
2. Use viewport units (vw) for fluid images
3. Use fixed pixels for constrained images
4. Order conditions from narrowest to widest

---

Accessibility

Alt Text Requirements

Every image needs meaningful alt text:

<!-- Informative: describe the content -->
<img alt="Team members collaborating around a whiteboard"/>

<!-- Decorative: empty alt -->
<img alt="" role="presentation"/>

<!-- Functional: describe the action -->
<img alt="Search"/>

Avoid Redundant Alt Text

<!-- Bad: redundant with adjacent text -->
<figure>
  <img alt="Photo of our office building"/>
  <figcaption>Our office building in Seattle</figcaption>
</figure>

<!-- Good: complementary description -->
<figure>
  <img alt="Modern glass-walled building with green roof"/>
  <figcaption>Our office building in Seattle</figcaption>
</figure>

---

Checklist

When adding images:

• Include alt attribute (descriptive or empty for decorative)
• Add loading attribute (eager for LCP, lazy for others)
• Add decoding="async" for all images
• Add fetchpriority="high" for LCP/hero images
• Include width and height to prevent layout shift
• Use srcset for resolution switching when image > 400px
• Calculate appropriate sizes based on rendered width
• Use <picture> for format fallbacks (AVIF > WebP > JPEG)
• Use <picture> for art direction (different crops)
• Test with network throttling to verify lazy loading

Related Skills

• css-author - Container queries for component-scoped image sizing
• performance - Write performance-friendly HTML pages
• xhtml-author - Write valid XHTML-strict HTML5 markup
• placeholder-images - Generate SVG placeholder images for prototypes