Media queries of <source> elements can be used only within <picture> elements

TL;DR: You can set the media attribute of a <source> element only within <picture> elements. So you can't switch a <video> source based on the preferred color scheme for dark/light mode support.

A neat use case of the media attribute on a <source> element is to toggle images based on the preferred colors scheme.

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

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

A nice plus of this approach is that there won't be any waste of bandwidth: the browser will download only the image associated with the current theme.

Unfortunately, you can't use the media attribute on <video> sources — it would get ignored in most major browsers.

<!-- Won't work -->
<video>
  <source
    src="dark.mp4"
    type="video/mp4"
    media="(prefers-color-scheme: dark)"
  />
  <source src="light.mp4" type="video/mp4" />
</video>

<!-- Won't work -->
<video>
  <source
    src="dark.mp4"
    type="video/mp4"
    media="(prefers-color-scheme: dark)"
  />
  <source src="light.mp4" type="video/mp4" />
</video>

So you'll still need to do it with some CSS (or JS).

<video class="dark" src="dark.mp4" />
<video class="light" src="light.mp4" />
 
<style>
  .light {
    display: block;
  }
 
  .dark {
    display: none;
  }
 
  @media (prefers-color-scheme: dark) {
    .light {
      display: none;
    }
 
    .dark {
      display: block;
    }
  }
</style>

<video class="dark" src="dark.mp4" />
<video class="light" src="light.mp4" />
 
<style>
  .light {
    display: block;
  }
 
  .dark {
    display: none;
  }
 
  @media (prefers-color-scheme: dark) {
    .light {
      display: none;
    }
 
    .dark {
      display: block;
    }
  }
</style>