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How to change an HTML image URL in dark mode

Published Apr 29 2020

Using CSS it’s pretty easy to apply changes if the system is in dark mode, using the prefers-color-scheme media feature.

Check my blog post on dark mode if you want to learn more about it.

Today I run into a problem - how to change an image defined in the HTML, rather than a CSS rule?

Turns out there’s a plain HTML way to do that, without any CSS or JavaScript involved.

We can use the picture tag to wrap the img tag:

<picture>
  <source 
    srcset="dark.png" 
    media="(prefers-color-scheme: dark)">
  <img src="light.png">
</picture>

If dark mode is supported and enabled, the dark.png image will be used as the source for the img tag.

The tag is very well supported, and old browsers that do not implement it, or do not implement dark mode, will fall back to displaying the light.png image.

It’s important to note that the browser does not download 2 images, in any case: if it’s dark mode, in this example it will just download the dark.png image, and if it’s light mode, it will download only light.png, so there’s no waste of bandwidth.