The preload attribute provides a hint to the browser as to how much downloading is sensible for a given media. The preload attribute can have three values:
- "none" - suggests to the browser that it doesn't need to load this media at all until the user plays the resource. The browser will delay any network traffic required to load the media until the users tries to play the resource, or explicitly loads the resource. I suggest using this preload value, along with the poster attribute, when it's unlikely that the user will play the resource. This is probably most useful in a mobile environment, where data can be expensive.
- "metadata" - suggests to the browser that it isn't necessary to load the entire resource in advance. The browser will suspend the load after loading metadata, displaying the first video frame (if there's no poster image), and ensuring it can play the media. This is the default behaviour, and prevents excess network traffic when the web developer isn't certain the video will definitely be played. If you don't specify a preload value, Firefox will automatically do this. This was also the default behaviour in Firefox 3.5 and 3.6 in the absence of the autobuffer attribute. This default behaviour is a suitable compromise between bandwidth saving and user convenience.
- "auto" - suggests to the browser that it should load as much of the resource as possible. As long as the browser's own media cache isn't full, it will keep on downloading. I suggest this is most useful in the "YouTube" case, when you've got a media which the user is almost certainly going to watch, and so having the user download the media is not likely to be wasting server bandwidth. This behaviour is the same as using the autobuffer attribute in Firefox 3.5 and 3.6
<video autobuffer preload="auto" src="video.ogg"></video>
If you want the video to only be downloaded if the user actually plays the video, you should not include either the preload or the autobuffer attribute. The default behaviour in Firefox 3.5, 3.6, and the upcoming Firefox 4 is to only load up to the first frame and then suspend the download.
"This is probably most useful in a mobile environment"
Well, I disagree on the side that if I'm in a mobile (or any other network-constrained) environment or not should be a decision on the browser side and not on the side of the website, which usually can't reliably tell how expensive data and network traffic is on my client.
kairo: it's an authoring hint that browsers are free to overrule as they see fit, which will happen in particular in network-constrained situations. I see the attribute most useful on pages where I have a lot of videos to display, but don't really want all of the video tags to be loading the metadata or more because that would be overkill for the page.
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