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Client hints

Peakhour uses client hints to provide optimized images based on the capabilities of the client's device. Client hints are additional headers that the browser sends to the server that request information about the client's runtime environment. By taking into account these hints, Peakhour can ensure that it provides the best possible images based on the client's device capabilities.

Supported Client Hints#

Peakhour currently supports the following client hints when transforming images:

  • DPR (Device Pixel Ratio)
  • Width
  • Save-Data

Implementing Client Hints#

Client hints can be implemented in two ways:

  1. By using a meta tag in the section of the HTML document:
<meta http-equiv="Accept-CH" content="DPR, Width, Viewport-Width">
  1. By including the Accept-CH header in the server's response:
Accept-CH: DPR,Width,Device-Memory,Content-DPR,Downlink,ECT,Save-Data,Viewport-Width,rtt

Sizes Attribute and Media Conditions#

In order to ensure that images are displayed correctly, the sizes attribute can be added to the tag to specify the size of the image based on a set of media conditions.

For example:

<img sizes="100vw" src="/images/sample.jpg"/>

The 100vw value in the sizes attribute tells the browser to assume that the image should fill the entire width of the browser window.

Media conditions, which are part of a media query, allow developers to specify conditions for different devices or viewport sizes. The following types of media conditions can be used:

  1. Plain condition: (min-width: 900px)
  2. Not condition: not (orientation: landscape)
  3. And condition: (orientation: landscape) and (min-width: 900px)
  4. Or condition: (orientation: portrait) or (max-width: 500px)

Automating with Lazysizes#

Lazysizes is a popular library that can be used to automatically calculate the sizes attribute for responsive images. The library can be found on GitHub at

<img data-src="image.jpg" data-sizes="auto" class="lazyload" />

By including the data-sizes="auto" attribute, Lazysizes will automatically calculate the correct sizes attribute based on the client's device capabilities.