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Repository to curated best practices for web images 🏞

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// repository documentation

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# Best practices for images

Repository to curated best practices for web images

> This repository focuses on Web Performance best practices for images. Performance optimization does not replace accessibility: attributes like `alt` are essential and are out of scope for this guide.

## Responsive images

### With media

We can use `media` to define a media query as a breakpoint to load a responsive image.

```html
<picture>
  <source srcset="image-ultrawide.jpg" media="(min-width: 1200px)">
  <source srcset="image-wide.jpg" media="(min-width: 600px)">
  
  <img src="image.jpg" height="300" width="200" alt="Awesome image">
</picture>
```

> In the above code, the browser loads the `image.jpg` in mobile version, the `image-wide.jpg` in tablets resolutions or bigger, and the `image-ultrawide.jpg` will be loaded in screen resolutions bigger than 1200px.
> NOTE: Browser finds the first matching condition and ignores everything after.

### With sizes

We can use [`sizes`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/sizes), that allows you to specify the layout width of the image for a list of media conditions.

```html
<img
  srcset="image-wide.jpg 600w,
          image-ultrawide.jpg 1200w"
  sizes="(min-width: 600px) 600w,
         (min-width: 1200px) 1200w"
  src="image.jpg" height="300" width="200" alt="Awesome image">
```

> In the above code (like with `media`), the browser loads the `image.jpg` in mobile version, the `image-wide.jpg` in tablets resolutions or bigger, and the `image-ultrawide.jpg` will be loaded in screen resolutions bigger than 1200px.

## Serve modern image formats

> Usually, the next-gen image formats like [`WebP`](https://developers.google.com/speed/webp), [`AVIF`](https://aomediacodec.github.io/av1-avif/) or [`JPEG XL`](https://jpeg.org/jpegxl/), can optimize image weight while maintaining good image quality.

> [`JPEG XL`](https://jpeg.org/jpegxl/) offers the best compression of all modern formats and is worth watching closely. As of 2025, support is available in Safari and Firefox (experimental). Chrome removed support in 2022 but the discussion remains open. Including it as the first `<source>` is safe: browsers that don't support it will simply fall through to the next format.

With the HTML tag `<picture>` we can specify the `type` in the `<source>` tag. The type is the image format, and we can use it to serve modern image formats. The browser will use the first image format that it supports.

```html
<picture>
  <source type="image/webp" srcset="image.webp">
  
  <img src="image.jpg" height="300" width="200" alt="Awesome image">
</picture>
```

> In the above code, the browser loads the first image format that it can render. E.g. Internet Explorer 11 nor Safari 13 can't load the [WebP](https://developers.google.com/speed/webp) image format (a next-gen image format), so they will load the `JPEG` image.

```html
<picture>
  
  <source type="image/jxl" srcset="image.jxl">
  <source type="image/avif" srcset="image.avif">
  <source type="image/webp" srcset="image.webp">
  
  <img src="image.jpg" height="300" width="200" alt="Awesome image">
</picture>
```

> In the above code, we list modern image formats from "more optimized" to "less optimized". The browser will show the first image that it can load and render.

## Serve responsive and modern image formats

We can combine both approaches to serve modern image formats and responsive images to load the best image on each device.

```html
<picture>
  
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.jxl 600w,
            image-ultrawide.jxl 1200w"
    type="image/jxl">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.avif 600w,
            image-ultrawide.avif 1200w"
    type="image/avif">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.webp 600w,
            image-ultrawide.webp 1200w"
    type="image/webp">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.jpg 600w,
            image-ultrawide.jpg 1200w"
    type="image/jpeg">
  
  <img src="image.jpg" height="300" width="200" alt="Awesome image">
</picture>
```
> In the above code, we have a combination of all modern image formats and the sizes needed. This is an example, as every site or project needs different sizes.

## Improve the Web Performance

We can use several attributes to improve the Web Performance, aka **user experience**.

- [`loading`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/loading) provides a hint to the user agent on how to handle the loading of the image which is currently outside the window's visual viewport. We can set it to `eager` (default value) which tells the browser to load the image as soon as the `<img>` element is processed, or `lazy` that tells the user agent to hold off on loading the image until the browser estimates that it will be needed imminently.
- [`decoding`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/decoding) represents a hint given to the browser on how it should decode the image. The values are `sync` to decode the image synchronously for atomic presentation with other content, `async` to decode the image asynchronously and reduce delay in presenting other content, and `auto` (default mode) which indicates no preference for the decoding mode. The browser decides what is best for the user.
- [`fetchpriority`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/fetchPriority) represents a hint given to the browser on how it should prioritize the fetching of the image relative to other resources. The values are `high` to fetch the image at a high priority relative to other images, `low` to fetch the image at a low priority relative to other images, and `auto` (default mode) which indicates no preference for the fetch priority. The browser decides what is best for the user. By default images are initially `low`, until the browser has done layout and knows what images are in the viewport. These are then boosted to `high` priority. You can make this boost happen earlier for an important image (e.g. LCP image) you know has a high probability of being in the viewport by using `fetchpriority="high"`. Similarly you can deprioritize less important images that are technically in the viewport, but not viewable (e.g. second and third carousel images) by using `fetchpriority="low"`. Typically you would not need to use fetchpriority on off-screen images as the browser default (low until in viewport) is optimal there.

### Now we will use this attributes to improve the user experience of our images

```html
<picture>
  
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.jxl 600w,
            image-ultrawide.jxl 1200w"
    type="image/jxl">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.avif 600w,
            image-ultrawide.avif 1200w"
    type="image/avif">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.webp 600w,
            image-ultrawide.webp 1200w"
    type="image/webp">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.jpg 600w,
            image-ultrawide.jpg 1200w"
    type="image/jpeg">
  
  <img
    loading="lazy"
    decoding="async"
    src="image.jpg"
    height="300"
    width="200"
    alt="Awesome image">
</picture>
```

> In the above code, we have a better code for all the images below the fold (outside the viewport) and the browser does not load these images (according to the [threshold](https://github.com/chromium/chromium/blob/main/third_party/blink/renderer/core/frame/settings.json5#L936-L968))

### Improve the image detected as [LCP](https://web.dev/lcp) element of the Core Web Vitals

The previous code covers the scenario for all images outside of the viewport. Usually, the [LCP](https://web.dev/lcp) metric refers to an image element, so we can iterate to improve it.

```html
<picture>
  
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.jxl 600w,
            image-ultrawide.jxl 1200w"
    type="image/jxl">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.avif 600w,
            image-ultrawide.avif 1200w"
    type="image/avif">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.webp 600w,
            image-ultrawide.webp 1200w"
    type="image/webp">
  <source
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="image-wide.jpg 600w,
            image-ultrawide.jpg 1200w"
    type="image/jpeg">
  
  <img
    fetchpriority="high"
    decoding="sync"
    src="image.jpg"
    height="300"
    width="200"
    alt="Awesome image">
</picture>
```

> In the above code, we changed the attribute `decoding` to `sync` to prioritize the decoding, removed the attribute `loading` because the default behavior is `eager`, so we don't need it, and we add the attribute `fetchpriority="high"` to indicate to the browser to load the image as soon as possible.

> By the way, we don't need to add these attributes to all `<source>` tags, only to `<img>` tags.

#### Preload the LCP image

`fetchpriority="high"` boosts the download priority, but only once the browser has parsed the `<img>` tag in the HTML. If the LCP image is discovered late — inside a component, a carousel, or injected via JavaScript — the browser may start downloading it too late.

A `<link rel="preload">` in the `<head>` solves this: it starts the download before the parser reaches the image.

```html

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

If you use `srcset` with modern formats, the preload tag supports `imagesrcset` and `imagesizes` so the browser preloads exactly the resource it will use:

```html
<link
  rel="preload"
  as="image"
  imagesrcset="image-wide.avif 600w, image-ultrawide.avif 1200w"
  imagesizes="(min-width: 600px) 100vw, 600px"
  fetchpriority="high">
```

> See [Optimize when the resource is discovered](https://web.dev/optimize-lcp/#optimize-when-the-resource-is-discovered) for more details.

## Use a CDN Image Service

We see that we need a lot of code to deliver the best user experience 🙈.

We can use a CDN Image Service like [Cloudinary](https://cloudinary.com/) to remove the part of code that references the image format, as these services serve the best image format supported by the browser.

The browser sends in the [HTTP Headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers) the [`accept`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept) header value to indicate which content types, expressed as [MIME types](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types).

![Accept Header](https://raw.githubusercontent.com/nucliweb/image-element/main/images/accept.png)

> In the screenshot above we see that my current version browser supports `image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;`, so with a service of automatic image format, the service will send me an `avif` image format in response.

```html
<picture>
  <img
    sizes="(min-width: 600px) 100vw, 600px,
           (min-width: 1200px) 100vw, 1200px"
    srcset="
      https://res.cloudinary.com/nucliweb/image/upload/c_scale,f_auto,w_600/v1010101010/demo/image.png 600px,
      https://res.cloudinary.com/nucliweb/image/upload/c_scale,f_auto,w_1200/v1010101010/demo/image.png 1200px"
    src="https://res.cloudinary.com/nucliweb/image/upload/f_auto/v1010101010/demo/image.png"
    loading="lazy"
    decoding="async"
    height="300"
    width="200"
    alt="Awesome image">
</picture>
```

> Now we don't need to define the type because this is transparent to us. The service sends the best image format supported by the browser. Notice that the image in the samples is an `PNG` image, yet the browser will load an `avif` image format.

<hr>

## Acknowledgement

- [Addy Osmani](https://github.com/addyosmani), for this [image](images/image-addy-osmani.jpg) and for all the tips that he are sharing with the community.

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