Why does the finding sometimes say 0 KB?

0 KB means the browser reported 0 bytes for this video file to the measurement. This happens when the transfer is still running at the end of the measurement, or when the video comes from a third-party server that omits the Timing-Allow-Origin header. The finding stands anyway, because autoplay triggers the loading.

Is a muted video more economical?

The muted attribute switches the sound off; the audio track still loads. The file gets smaller if you remove the audio track when compressing. The picture makes up the largest part of the file.

Does an embedded YouTube video count?

This rule reads the video elements of the page. What an embedded player of a third-party provider loads counts in the page weight.

This translation has not been editorially reviewed yet. The German version is authoritative. Deutsche Fassung

CO₂ & EmpCo

How much does a video that starts on its own cost?

A background video in the header of the home page is usually the largest file of the whole page. If it starts by itself, it loads on every visit, also for visitors who scroll on straight away. The check reports every video with autoplay, also with preload="none", with its address, the KB transferred and whether a style sheet of the page takes reduced motion into account.

What the check reads

It looks for video elements with the autoplay attribute and counts the KB of the video file transferred during the page view. If preload="none" is also set, the finding says so explicitly (“despite preload="none"”). It also checks whether a style sheet of the page contains a rule for prefers-reduced-motion; style sheets on third-party servers count too if their server allows reading via CORS. The finding states this as “respected” or “ignored”. “Respected” means such a rule exists in the style sheet; whether it applies to the video is shown by a look at the rule.

Motion and accessibility

People who have set “reduce motion” in their system often react to moving backgrounds with dizziness or find it hard to concentrate. A page can show them a still image and save the video file entirely: a small script starts the video only with the normal motion setting. If moving content plays by itself for longer than five seconds, WCAG 2.2 also requires a way to pause it (criterion 2.2.2).

What autoplay does to preload

preload="none" asks the browser to wait with loading until playback. If autoplay is set at the same time, playback starts immediately and the browser loads. A video is therefore economical only when it starts on a click: with controls, preload="none" and a preview image (poster).

Examples

Background video

Not met

<video autoplay muted loop src="/medien/hintergrund.mp4"></video>

Passes

<video controls preload="none"
       poster="/medien/hintergrund-standbild.webp">
  <source src="/medien/hintergrund.mp4" type="video/mp4">
</video>

Decide whether the video shows information or sets a mood. For mood, a still image is often enough.
Start the video on a click: remove autoplay, set controls and preload="none", and give a preview image as poster.
If a background video with autoplay stays, the finding stays too. Then keep the file small (short loop, low resolution, audio track removed) and show a still image under prefers-reduced-motion.
Then order a re-measurement to see the new page weight.

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