Remember Client Hints?

In 2014 Google Chrome brought us all Client Hints, an exciting way to pass image width and DPR in a browser's request headers. This made it much easier to serve responsive images by knowing the correct size needed for an image. Except one issue... it caused security concerns and was removed in 2017. This has been cleared up with the setting of a Feature Policy and you can now use Client Hints again! Here is some info from Google about Client Hints as well.

Client Hints does work on about 74% of the web right now. Is it still worth it? Yes! Looking back at some previous studies, Smashing Mag determined that it saved 19-32% on bandwidth than using predetermined image breakpoints in srcset.

Using Client Hints

There are a few image CDNs that support using Client Hints. I work at imgix, so naturally I have chosen to use imgix for this example. imgix is an API based image management, optimization, & delivery service. To use Client Hints with an image on imgix, you would simply add ch=width, dpr to that image. It also supports save-data, but I'm not using that for this example.

In this example, I've added the ch=width,dpr for client hints, then w=400 as the fallback size option for browsers that don't support it like Firefox or Safari. It's also important to add the sizes; in my example below I'm not using any css so this determines what size my images should be.

<img src="https://imgix.tomdale.website/artsy/6.jpg?ch=width,dpr&w=400"
sizes="(min-width: 768px) 30vw, 100vw" />

Let's look at a Demo

I created a simple demo site to show this working. It's important to note that I tried to make this site as simple as possible with minimal css to ensure any changes to the images were purely happening because of the Client Hints. Please note that if you open a specific image in a separate tab by itself, it will not appear the same size as used on the website. It only sizes according to Client Hints when on the actual site.

I've included 9 example images. Each image code is like this:

<img src="https://imgix.tomdale.website/artsy/5.jpg?ch=width,dpr&w=400&ar=4:3&fit=crop&crop=faces,edges&auto=format,compress" 
sizes="(min-width: 768px) 30vw, 100vw" />

While in a supporting browser, like Chrome, the images will we 30% the width of the browser width when the browser is 768 pixels or larger. Once it goes smaller than that, each image will be sized at 100% the width of the browser. This is truly a dynamic size, every time you resize this browser and do a hard refresh you will notice the weight of the images change in the network tab.

In an ideal world, you should also be using srcset in combination with Client Hints. You do want to serve a good size image for the other 25% of users that aren't supported for Client Hints. In this demo I did not do that because I wanted to focus on showing the size of these images. You will also notice the cropping points of these images change a bit, that's because I am using some imgix API to automatically crop to a face first, then a prominent object in the photo as a fallback option if there are no faces.

Deploying to Vercel

I enjoy using both Vercel & Netlify to deploy my sites. They both have a lot of benefits. In this example I did use Vercel. The important item needed to make Client Hints work is setting a feature policy. With Vercel, you can set that policy by adding a vercel.json file. For my demo I put this in the vercel.json file:

{
  "routes": [
    {
      "src": "/*",
        "headers": {
          "Accept-CH": "DPR, Width, Viewport-Width",
          "Feature-Policy": "ch-dpr https://imgix.tomdale.website 'self'; ch-width https://imgix.tomdale.website 'self'; ch-viewport-width https://imgix.tomdale.website 'self'"
        }
    }
  ]
}

You should change that url to the url you are using for your images to make this work. But, in order to serve Client Hints safely, this isn't that big of an effort to add.

Conclusion

It's great to see Client Hints working again. It's still not the 100% solution you can use, so you do still need to follow responsive image best practices with setting srcsets for different image breakpoints. But Client Hints is certainly on the path to be the ideal solution in the future to serving those perfect responsive images. Here is a link the Github Repo for this project. Feel free to reach out to me if you have any other questions as well!