Responsive images and preloading don’t naturally work well together. When you use responsive images, the browser makes smart decisions about which image to load based on the device’s characteristics. But preloading needs to happen before these decisions can be made.
Preloading responsive images
Baseline Widely available
Supported in Chrome: yes.
Supported in Edge: yes.
Supported in Firefox: yes.
Supported in Safari: yes.
Supported in Edge: yes.
Supported in Firefox: yes.
Supported in Safari: yes.
This feature is well established and works across many devices and browser versions. It’s been available across browsers since December 2023
Here’s what a typical responsive image looks like:
<img srcset="small.jpg 300w, medium.jpg 600w, large.jpg 900w" sizes="(max-width: 768px) 100vw, (max-width: 1200px) 50vw, 900px" src="medium.jpg" alt="A responsive image" />The browser examines these srcset and sizes attributes to determine which image to download. It considers the viewport width, the device’s pixel density, the sizes attribute, and the available image widths. This smart selection process is what makes responsive images so powerful.
The common approach to preloading breaks down with responsive images.
<link rel="preload" href="medium.jpg" as="image" />This fails because it forces the browser to download a specific image variant without considering the responsive image selection logic. You might end up downloading the wrong size, wasting bandwidth and potentially hurting performance.
The Right Way: Responsive Preloading
Modern browsers support responsive preloading through imagesrcset and imagesizes attributes:
<link rel="preload" as="image" href="medium.jpg" imagesrcset="small.jpg 300w, medium.jpg 600w, large.jpg 900w" imagesizes="(max-width: 768px) 100vw, (max-width: 1200px) 50vw, 900px" />This approach tells the browser about all available image variants and how their widths should be calculated. The browser can then make the same smart decisions about which image to preload that it would make for a regular responsive image.
When working with modern image formats like WebP or AVIF, browser support becomes an important consideration:
<link rel="preload" as="image" href="photo.avif" imagesrcset="photo-300.avif 300w, photo-600.avif 600w, photo-900.avif 900w" imagesizes="(max-width: 768px) 100vw, (max-width: 1200px) 50vw, 900px" type="image/avif" />The type attribute prevents browsers that don’t support AVIF from downloading these images. This same concept applies when working with multiple image formats in your responsive image markup:
<picture> <source type="image/avif" srcset="photo-300.avif 300w, photo-600.avif 600w, photo-900.avif 900w" sizes="(max-width: 768px) 100vw, (max-width: 1200px) 50vw, 900px" /> <img src="photo-600.jpg" alt="Photo" /></picture>You can verify your responsive preloading setup using the browser’s DevTools and Performance Observer API:
const observer = new PerformanceObserver((list) => { const entries = list.getEntries(); entries.forEach((entry) => { if (entry.name.includes('photo')) { console.log(`Downloaded: ${entry.name}`); console.log(`Size: ${entry.transferSize} bytes`); console.log(`Time: ${entry.duration}ms`); } });});
observer.observe({ entryTypes: ['resource'] });This code helps you monitor which image variants are being downloaded and how long they take, giving you concrete data about your preloading strategy’s effectiveness.
Web performance is a balancing act. While preloading responsive images can significantly improve load times, it’s crucial to implement it thoughtfully.
Consider preloading only your most critical responsive images, typically those that appear above the fold or contribute to your Largest Contentful Paint (LCP).
By combining responsive image preloading with modern image formats, you’ll deliver the optimal image variant to each user while ensuring it loads as quickly as possible.
