New rule: Never preload video in the <video> tag

[JulienWilhelm]

When the <video> tag is used, the video source is loaded at the same time as the page. However, it is never certain that the user will consult this content.

To prevent downloading of the video, you must use the "preload" attribute with the value "none".

Please note that the use of "preload=none" should be supplemented with the "poster" attribute to provide an (optimized) preview image of the video content.

<!-- Bad: "preload" is not specified, so the video is automatically downloaded. -->
<video>
	<source src="/path/to/video.m4v" type="video/mp4">
</video>

<!-- Almost good: "preload=none" is specified, so the video will only be downloaded if the user asks to play it. But there is no preview. -->
<video preload="none">
	<source src="/path/to/video.m4v" type="video/mp4">
</video>

<!-- Good: "preload=none" is specified AND "poster" too -->
<video preload="none" poster="/path/to/poster.webp">
	<source src="/path/to/video.m4v" type="video/mp4">
</video>

Note: when the "autoplay" attribute is specified, "preload=none" will be ignored.