Responsive images

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Text on the web automatically wraps at the edge of the screen so that it doesn't overflow. Images, on the other hand, have an intrinsic size. If an image is wider than the screen, the image overflows and the user has to scroll horizontally to see all of it.

Fortunately, CSS gives you tools to stop this from happening.

Constrain your images

In your style sheet, you can use max-inline-size to declare that images can never be rendered at a size wider than their containing element.

img {
  max-inline-size: 100%;
  block-size: auto;
}

You can apply the same rule to other kinds of embedded content too, like videos and iframes.

img,
video,
iframe {
  max-inline-size: 100%;
  block-size: auto;
}

With this rule in place, browsers automatically scale down images to fit on the screen.

Adding a block-size value of auto means the browser preserves your images' aspect ratio as it resizes them.

Sometimes, an image's dimensions are set by a content management system (CMS) or other content delivery system. If your design calls for a different aspect ratio from the CMS's default, you can use the aspect-ratio property to preserve your site's design:

img {
  max-inline-size: 100%;
  block-size: auto;
  aspect-ratio: 2/1;
}

Unfortunately, this often means the browser has to squash or stretch the image to make it fit in the intended space.

To prevent squashing and stretching, use the object-fit property.

An object-fit value of contain tells the browser to preserve the image's aspect ratio, leaving empty space around the image if needed.

img {
  max-inline-size: 100%;
  block-size: auto;
  aspect-ratio: 2/1;
  object-fit: contain;
}

An object-fit value of cover tells the browser to preserve the image's aspect ratio, cropping the image if needed.

img {
  max-inline-size: 100%;
  block-size: auto;
  aspect-ratio: 2/1;
  object-fit: cover;
}

You can change the position of the image crop using the object-position property. This adjusts the focus of the crop, so you can make sure the most important part of the image is still visible.

img {
  max-inline-size: 100%;
  block-size: auto;
  aspect-ratio: 2/1;
  object-fit: cover;
  object-position: top center;
}

Deliver your images

Those CSS rules tell the browser how you'd like images to be rendered. You can also provide hints in your HTML about how the browser should handle those images.

Hints for sizing

If you know your image's dimensions, always include width and height attributes. Even if the image is rendered at a different size because of your max-inline-size rule, the browser still knows the width to height ratio and can set aside the right amount of space. This prevents your other content from jumping around when the image loads.

<img
 src="image.png"
 alt="A description of the image."
 width="300"
 height="200"
>

Hints for loading

Use the loading attribute to tell the browser whether to delay loading the image until it's in or near the viewport. For images below the fold, use a value of lazy. The browser won't load lazy images until the user has scrolled far down enough that the image is about to come into view. If the user never scrolls, the image never loads.

<img
 src="image.png"
 alt="A description of the image."
 width="300"
 height="200"
 loading="lazy"
>

For a hero image above the fold, don't use loading. If your site automatically applies the loading="lazy" attribute, you can usually set loading to the default value of eager to prevent images from being lazy loaded:

<img
 src="hero.jpg"
 alt="A description of the image."
 width="1200"
 height="800"
 loading="eager"
>

Fetch Priority

For important images-such as the LCP image, you can further prioritize the loading using Fetch Priority by setting the fetchpriority attribute to high:

<img
 src="hero.jpg"
 alt="A description of the image."
 width="1200"
 height="800"
 loading="eager"
 fetchpriority="high"
>

This tells the browser to fetch the image right away and at high priority, instead of waiting until the browser has finished its layout and would fetch images normally.

However, when you ask the browser to prioritize downloading one resource, like an image, the browser must de-prioritize another resource such as a script or a font file. Only set fetchpriority="high" on an image if it's truly vital.

Hints for preloading

It's best to avoid preloading whenever possible by including all images in the initial HTML file. However, some images may be unavailable, such as images added by JavaScript or a CSS background image.

You can use preloading to get the browser to fetch these important images ahead of time. For really important images, you can combine this preloading with the fetchpriority attribute:

<link rel="preload" href="hero.jpg" as="image" fetchpriority="high">

Again, use these attributes sparingly to avoid overriding the browser's prioritization heuristics too often. Overusing them can cause performance degradation.

Some browsers support preloading responsive images based on srcset, using the imagesrcset and imagesizes attributes. For example:

<link rel="preload" imagesrcset="hero_sm.jpg 1x hero_med.jpg 2x hero_lg.jpg 3x" as="image" fetchpriority="high">

By excluding the href fallback, you can make sure browsers without srcset support still preload the correct image.

You can't preload images in different formats based on browser support of certain formats. Attempting this can result in extra downloads that waste users' data.

Image decoding

There's also a decoding attribute you can add to img elements. You can tell the browser that the image can be decoded asynchronously, so it can prioritize processing other content.

<img
 src="image.png"
 alt="A description of the image."
 width="300"
 height="200"
 loading="lazy"
 decoding="async"
>

You can use the sync value if the image itself is the most important piece of content to prioritize.

<img
 src="hero.jpg"
 alt="A description of the image."
 width="1200"
 height="800"
 loading="eager"
 decoding="sync"
>

The decoding attribute doesn't change how fast the image decodes. It affects only whether the browser waits for this image decoding to happen before rendering other content.

In most cases this doesn't have much impact, but sometimes it can let the browser display your image or other content slightly faster. For example, for a large document with lots of elements that take time to render, and with large images that take a long time to decode, setting sync on important images tells the browser to wait for the image and render both at once. Alternatively, you can set async to let the browser display content faster and without waiting for the image to decode.

However, the better option is usually to try to avoid excessive DOM sizes and use responsive images to reduce decoding time, instead of using decoding.

Responsive images with srcset

Thanks to that max-inline-size: 100% declaration, your images can't break out of their containers. However, if a user has a small screen and a low-bandwidth network, making them download the same size images as users with larger screens wastes data.

To fix this issue, add multiple versions of the same image at different sizes, and use the srcset attribute to tell the browser these sizes exist and when to use them.

Width descriptor

You can define a srcset using a comma-separated list of values. Each value is the URL of an image, followed by a space, followed by some metadata about the image, called a descriptor.

In this example, the metadata describes the width of each image using the w unit. One w is the width of one pixel.

<img
 src="small-image.png"
 alt="A description of the image."
 width="300"
 height="200"
 loading="lazy"
 decoding="async"
 srcset="small-image.png 300w,
  medium-image.png 600w,
  large-image.png 1200w"
>

The srcset attribute complements the src attribute instead of replacing it. You still need to have a valid src attribute, but the browser can replace its value with one of the options listed in the srcset. To save bandwidth, the browser only downloads the larger image if they're needed.

Sizes

If you're using the width descriptor, you must also use the sizes attribute to give the browser more information. This tells the browser what size you expect the image to be displayed at under different conditions. Those conditions are specified in a media query.

The sizes attribute takes a comma-separated list of media queries and image widths.

<img
 src="small-image.png"
 alt="A description of the image."
 width="300"
 height="200"
 loading="lazy"
 decoding="async"
 srcset="small-image.png 300w,
  medium-image.png 600w,
  large-image.png 1200w"
 sizes="(min-width: 66em) 33vw,
  (min-width: 44em) 50vw,
  100vw"
>

In this example, you're telling the browser that in a viewport with a width over 66em, it should display the image no wider than one third of the screen (inside a three-column layout, for example).

For viewport widths between 44em and 66em, display the image at half the width of the screen (as in a two-column layout).

For anything narrower than 44em, display the image at the full width of the screen.

This means that the biggest image won't necessarily be used for the widest screen. A wide browser window that can display a multi-column layout uses an image that fits in one column, which might be smaller than an image used for a single-column layout on a narrower screen.

Pixel density descriptor

You can also use descriptors to provide an alternate version of images to show on high-density displays, to keep images looking sharp at the higher resolutions they provide.

Use the density descriptor to describe the pixel density of the image in relation to the image in the src attribute. The density descriptor is a number followed by the letter x, as in 1x or 2x.

<img
 src="small-image.png"
 alt="A description of the image."
 width="300"
 height="200"
 loading="lazy"
 decoding="async"
 srcset="small-image.png 1x,
  medium-image.png 2x,
  large-image.png 3x"
>

If small-image.png is 300 by 200 pixels in size, and medium-image.png is 600 by 400 pixels, then medium-image.png can have 2x after it in the srcset list.

You don't have to use whole numbers. If another version of the image is 450 by 300 pixels in size, you can describe it with 1.5x.

Presentational images

Images in HTML are content. That's why you include the alt attribute with a description of the image for screen readers and search engines.

If you embed an image that's decorative, without any meaningful content, you can use an empty alt attribute.

<img
 src="flourish.png"
 alt=""
 width="400"
 height="50"
>

You must always include the alt attribute, even if it's empty. An empty alt attribute tells a screen reader that the image is presentational. A missing alt attribute doesn't provide that information.

Ideally, presentational or decorative images are included with CSS instead of HTML. HTML is for structure. CSS is for presentation.

Background images

Use the background-image property in CSS to load presentational images.

element {
  background-image: url(flourish.png);
}

You can specify multiple image candidates using the image-set function for background-image.

The image-set function in CSS works a lot like the srcset attribute in HTML. Provide a list of images with a pixel density descriptor for each one.

element {
  background-image: image-set(
    small-image.png 1x,
    medium-image.png 2x,
    large-image.png 3x
  );
}

The browser chooses the most appropriate image for the device's pixel density.

There are many factors to consider when you're adding images to your site, including:

• Reserving the right space for each image.
• Figuring out how many sizes you need.
• Deciding whether the image is content or decorative.

It's worth spending the time to get your images right. Poor image strategies can annoy and frustrate your users. A good image strategy makes your site feel snappy and sharp, regardless of the user's device or network connection.

There's one more HTML element in your toolkit to give you more control over your images: the picture element.

Check your understanding

Test your knowledge of images.