responsive-images（响应式图像）

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

Lazy Loading Rules

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

object-fit Values

Format Comparison

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