Better responsive images with srcset and sizes attributes

Introduction

srcset and sizes are two HTML attributes that can be used to create responsive images. They are used to specify the image source and the size of the image. They are used together to create images that are optimized for the device they are being displayed on.

In this article, I will not be going into the details of how responsive images work. I will be focusing on how to use the srcset and sizes attributes.

Let's take a look at the basic img element:

<img src="image.jpg" alt="image" />

The src attribute is used to specify the image source. Web browsers will download the image and display it in all devices with the same size no matter what the device's screen size, pixel density, or viewport size is.

So if you have a 2000px wide image, it will be displayed as a 2000px wide image on a 4K monitor - which is fine, but it will also be downloaded and displayed as a 2000px wide image on a 320px wide mobile phone screen - of course, it will fit the screen, but it's unnecessarily large and will take a long time to download.

That's where the srcset and sizes attributes come in. We will use them to provide different image sources/sizes for different devices and let the browser decide which image to download and display.

<img
  src="image.jpg"
  srcset="image-320.jpg 320w, image-640.jpg 640w, image-1280.jpg 1280w"
  sizes="(max-width: 320px) 280px, (max-width: 640px) 640px, 1280px"
  alt="image"
/>

The srcset and sizes values look a bit complicated (and easy to forget ), but they are not that hard to understand.

srcset

The srcset attribute is used to specify the image sources and their sizes. The image sources are separated by commas, and each image source is followed by its size in pixels with the following parts:

image-source - the image's URL (e.g. image-320.jpg)
A space
image-size - the image's intrinsic size in pixels (e.g. 320w) - notice the w at the end of the size instead of px to indicate that the size is in pixels.

In the example above, we have three image sources: image-320.jpg, image-640.jpg, and image-1280.jpg.

The first image source is image-320.jpg and it's 320px wide.
The second image source is image-640.jpg and it's 640px wide.
The third image source is image-1280.jpg and it's 1280px wide.

So now we have a set of images with different sizes, but how do we tell the browser which image to use ?

Here comes the sizes attribute.

sizes

The sizes attribute defines a set of media conditions and help the browser decide which image to use when the conditions are met.

Each size is separated by commas, and being constructed with the following parts:

A media condition - a set of media features and values that define the condition (e.g. (max-width: 320px)). Notice that the condition is wrapped in parentheses, like a css media query. In this case, the condition is that the viewport's width is less than or equal to 320px.
A space
A size - the size of the image to use when the condition is met (e.g. 280px).

And here are the steps of how the browser decides which image to use:

Looks at the device's screen size.
Looks at the sizes attribute and finds the first condition that matches the device's screen size.
Uses the size defined in the condition to find the image source with the same size in the srcset attribute, if there isn't one, it will use the first image that is larger than the size defined in the condition.
Load the image and display it.

And that's it, we have created a responsive image .

Take a look at the example above, let's say we are on a mobile phone with a screen size of 400px wide.

The first condition match that screen is (max-width: 640px).
The size defined in the condition is 640px.
The image source with the same size in the srcset attribute is image-640.jpg.
The browser will load the image and display it.

Beyond the basics

For the sizes attribute, you can use vw instead of px to define the size. This is useful when you want to use the viewport width as the size. For example, if you want to use the viewport width as the size, you can use 100vw as the size.
Combine srcset & sizes with loading="lazy" to improve performance. When the loading attribute is set to lazy, the browser will not load the image until it is visible in the viewport. This is useful when you have a lot of images on the page and you want to improve the page's performance.
Better add width and height attributes to the img element. When the width and height attributes are added to the img element, the browser will reserve the space for the image before it is loaded. This is useful to prevent layout shifts and improve the experience to your visitors.
And don't forget to add a fallback image with the src attribute. When the browser doesn't support the srcset and sizes attributes, it will use the image source defined in the src attribute.

At the end, the img element should look like this:

<img
	src="image.jpg"
	srcset="image-320.jpg 320w, image-640.jpg 640w, image-1280.jpg 1280w"
	sizes="(max-width: 320px) 280px, (max-width: 640px) 640px, 1280px"
	loading="lazy"
	alt="image alt text"
	width="1280"
	height="720"
/>

Happy coding .