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Table of Contents
What is srcset?
The meaning of the sizes attribute
Suggestions in actual use
Don't forget fallback
Home Web Front-end HTML Tutorial How to use the srcset and sizes attributes for responsive images?

How to use the srcset and sizes attributes for responsive images?

Jul 09, 2025 am 01:36 AM

srcset and sizes are key properties used in HTML for responsive images. 1. srcset provides multiple image versions for the browser to choose, such as pictures of different widths or pixel density; 2. sizes tell the browser the display width of the picture under different screen conditions, helping the browser to select the most suitable image resource based on the viewport size and device pixel ratio; 3. When using it, you should prepare multiple size images, clearly named, test device behavior, pay attention to performance trade-offs, and retain fallback's src images to ensure compatibility and default display effects.

How to use the srcset and sizes attributes for responsive images?

In responsive web design, srcset and sizes are two very useful HTML attributes. They help browsers select the most appropriate image resources based on device characteristics, thereby improving loading speed and display effects. Simply put, if you use these two attributes correctly, the pictures can automatically adapt to different screen sizes, which is both clear and efficient.

How to use the srcset and sizes attributes for responsive images?

What is srcset?

The purpose of srcset is to tell the browser what optional image resources are available. You can provide multiple resolution versions for a picture and label their width or pixel density.

For example:

How to use the srcset and sizes attributes for responsive images?
 <img src="/static/imghw/default1.png"  data-src="image.jpg"  class="lazy"
     srcset="image-320w.jpg 320w,
             image-480w.jpg 480w,
             image-800w.jpg 800w"
     alt="Example Picture">

In the example above:

  • Each file is followed by its width (indicated by w)
  • The browser will select the most suitable image based on the current viewport size, device pixel ratio and other information.

You can also use x to represent pixel density, for example:

How to use the srcset and sizes attributes for responsive images?
 srcset="image-hd.jpg 2x, image.jpg 1x"

Suitable for switching clearer pictures when high-resolution screens such as Retina.


The meaning of the sizes attribute

sizes tells the browser: under certain conditions, how wide the image will be. This helps the browser decide which image resource to choose from srcset .

For example:

 <img src="/static/imghw/default1.png"  data-src="image.jpg"  class="lazy"
     srcset="image-320w.jpg 320w,
             image-480w.jpg 480w,
             image-800w.jpg 800w"
     sizes="(max-width: 600px) 100vw,
            800px"
     alt="responsive picture">

This code means:

  • When the screen width is less than or equal to 600px, the image will occupy the entire viewport width (100vw)
  • Otherwise, the image width is fixed to 800px

Combining the multiple width options provided by srcset , the browser can determine which image should be loaded.

Note: sizes is only valid if srcset contains the width descriptor (w).


Suggestions in actual use

If you plan to use srcset and sizes , here are a few practical suggestions:

  • Prepare images of multiple sizes : Don't just upload one large image and zoom in on the browser. Generate several versions of common widths in advance, such as 320px, 768px, 1024px, etc.
  • Keep the naming clear : for example, image-320w.jpg and image-800w.jpg , so that errors are not prone to when writing srcset .
  • Test behavior under different devices : You can use browser developer tools to simulate different screen sizes and observe whether the correct picture is loaded.
  • Pay attention to performance trade-offs : Although multi-picture versions are flexible, you should also avoid providing too many unnecessary images to avoid increasing maintenance costs and server burden.

Don't forget fallback

Don't forget that src attribute is still important. It is both an alternative to old browsers and a default option when other conditions are not met. Make sure your src pictures are clear enough to handle most situations.


Basically that's it. Mastering srcset and sizes , you can make web pictures more smarter to adapt to various devices, rather than just loading large or blurry pictures.

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