Experimental responsive images

Type: boolean
Default: false

Dodane w: astro@5.0.0

Enables support for automatic responsive images in your project.

The term responsive images refers images that work well on different devices. This particularly applies to images that resize to fit their container, and that can be served in different sizes depending on the device’s screen size and resolution.

There are a number of additional properties that can be set to control how the image is displayed, but these can be complicated to handle manually. Incorrect handling of these properties can lead to images that are slow to download or that are not displayed correctly. This is one of the most common causes of poor Core Web Vitals and Lighthouse performance scores.

When this flag is enabled, Astro can automatically generate the required srcset and sizes values for images, and apply the correct styles to ensure they resize correctly. This behavior can be configured globally or on a per-image basis.

To enable the feature, first add the responsiveImages flag to your astro.config.mjs file:

astro.config.mjs

{
  experimental: {
    responsiveImages: true,
  },
}

Enabling this flag will not change anything by default, but responsive images can then be configured by setting the image layout either globally or per image.

To do this, you have access to additional image configuration settings for controlling the default behavior of all images processed and optimized by Astro:

• Local and remote images using the Markdown ![]() syntax.
• The <Image /> and <Picture /> components.

Additionally, Astro’s image components can receive responsive image props to override these defaults on a per-image basis.

Images in your public/ folder are never optimized, and responsive images are not supported.

Note

Enabling responsive images will generate additional image sizes for all affected images. For prerendered pages this happens during the build so may increase the build time of your project, especially if you have a large number of images.

For pages rendered on-demand the images are generated as-needed, so this has no impact on build times but may increase the number of transformations performed. Depending on your image service this may incur additional costs.

Image layout

Section titled Image layout

In order to generate the correct srcset and sizes attributes, the <Image /> and <Picture /> components need to know how the image should resize when its container changes size. This is done by setting the layout prop, or image.experimentalLayout default. The supported values are:

• constrained - The image will scale down to fit the container, maintaining its aspect ratio, but will not scale up beyond the specified width and height, or the image’s original dimensions. Use this if you want the image to display at the requested size where possible, but shrink to fit smaller screens. This matches the default behavior for images when using Tailwind. If you’re not sure, this is probably the layout you should choose.
• full-width - The image will scale to fit the width of the container, maintaining its aspect ratio. Use this for hero images or other images that should take up the full width of the page.
• fixed - The image will maintain the requested dimensions and not resize. It will generate a srcset to support high density displays, but not for different screen sizes. Use this if the image will not resize, for example icons or logos smaller than any screen width, or other images in a fixed-width container.
• none - The image will not be responsive. No srcset or sizes will be automatically generated, and no styles will be applied. This is useful if you have enabled a default layout, but want to disable it for a specific image.

The chosen layout will be used to generate the correct srcset and sizes attributes for the image, and will define the default styles applied to that <img> tag.

Configuration settings

Section titled Configuration settings

Set image.experimentalLayout with a default value to enable responsive images throughout your project.

If this value is not configured, you can still pass a layout prop to any <Image /> or <Picture /> component to create a responsive image. However, Markdown images will not be responsive.

Optionally, you can configure image.experimentalObjectFit and image.experimentalObjectPosition which will apply to all processed images by default.

Each of these settings can be overridden on any individual <Image /> or <Picture /> component with a prop, but Markdown images will always use the default settings.

astro.config.mjs

{
  image: {
    // Used for all Markdown images; not configurable per-image
    // Used for all `<Image />` and `<Picture />` components unless overridden with a prop
    experimentalLayout: 'constrained',
  },
  experimental: {
    responsiveImages: true,
  },
}

Responsive image properties

Section titled Responsive image properties

These are additional properties available to the <Image /> and <Picture /> components when responsive images are enabled:

• layout: The layout type for the image. Can be constrained, fixed, full-width, or none. If set to none, responsive behavior is disabled for this image and all other options are ignored. Defaults to none, or the value of image.experimentalLayout if set.
• fit: Defines how the image should be cropped if the aspect ratio is changed. Values match those of CSS object-fit. Defaults to cover, or the value of image.experimentalObjectFit if set.
• position: Defines the position of the image crop if the aspect ratio is changed. Values match those of CSS object-position. Defaults to center, or the value of image.experimentalObjectPosition if set.
• priority: If set, eagerly loads the image. Otherwise, images will be lazy-loaded. Use this for your largest above-the-fold image. Defaults to false.

The widths and sizes attributes are automatically generated based on the image’s dimensions and the layout type, and in most cases should not be set manually. The generated sizes attribute for constrained and full-width images is based on the assumption that the image is displayed at close to the full width of the screen when the viewport is smaller than the image’s width. If it is significantly different (e.g. if it’s in a multi-column layout on small screens) you may need to adjust the sizes attribute manually for best results.

The densities attribute is not compatible with responsive images and will be ignored if set.

For example, with constrained set as the default layout, you can override any individual image’s layout property:

---
import { Image } from 'astro:assets';
import myImage from '../assets/my_image.png';
---
<Image src={myImage} alt="This will use responsive layout" width={800} height={600} />
<Image src={myImage} alt="This will use full-width layout" layout="full-width" />
<Image src={myImage} alt="This will disable responsive images" layout="none" />

Generated HTML output for responsive images

Section titled Generated HTML output for responsive images

When a layout is set, either by default or on an individual component, images have automatically generated srcset and sizes attributes based on the image’s dimensions and the layout type. Images with constrained and full-width layouts will have styles applied to ensure they resize according to their container.

MyComponent.astro

---
import { Image, Picture } from 'astro:assets';
import myImage from '../assets/my_image.png';
---
<Image src={myImage} alt="A description of my image." layout='responsive' width={800} height={600} />
<Picture src={myImage} alt="A description of my image." layout='full-width' formats={['avif', 'webp', 'jpeg']} />

This <Image /> component will generate the following HTML output:

<img
  src="/_astro/my_image.hash3.webp"
  srcset="/_astro/my_image.hash1.webp 640w,
      /_astro/my_image.hash2.webp 750w,
      /_astro/my_image.hash3.webp 800w,
      /_astro/my_image.hash4.webp 828w,
      /_astro/my_image.hash5.webp 1080w,
      /_astro/my_image.hash6.webp 1280w,
      /_astro/my_image.hash7.webp 1600w"
  alt="A description of my image"
  sizes="(min-width: 800px) 800px, 100vw"
  loading="lazy"
  decoding="async"
  fetchpriority="auto"
  width="800"
  height="600"
  style="--fit: cover; --pos: center;"
  data-astro-image="constrained"
>

Overriding image styles

Section titled Overriding image styles

The responsive image component applies a small number of styles to ensure they resize correctly. The applied styles depend on the layout type, and are designed to give the best behavior for the generated srcset and sizes attributes. These are the default styles:

Responsive Image Styles

:where([data-astro-image]) {
  object-fit: var(--fit);
  object-position: var(--pos);
}
:where([data-astro-image='full-width']) {
  width: 100%;
}
:where([data-astro-image='constrained']) {
  max-width: 100%;
}

You can override the object-fit and object-position styles by setting the fit and position props on the <Image /> or <Picture /> component.

The styles use the :where() pseudo-class, which has a specificity of 0, meaning that it is easy to override with your own styles. Any class or tag name will have a higher specificity than :where(), so you can easily override the styles by adding your own class or tag name to the image.

Tailwind 4 is a special case, because it uses cascade layers, meaning the Tailwind rules are always lower specificity than rules that don’t use layers. Astro supports browsers that do not support cascade layers, so it cannot use them for images. This means that if you need to override the styles using Tailwind 4, you must use the !important modifier.

For a complete overview, and to give feedback on this experimental API, see the Responsive Images RFC.