HTML5video标签播放视频下载原理
HTML5 video
<video preload="metadata"> <!-- /i?id=676422 -->
<source src="assets/dizzy.mp4" type="video/mp4" />
<source src="assets/dizzy.webm" type="video/webm" />
<source src="v" type="video/ogg" />
</video>
HTML5⽀持video和audio之前，⽹页播放流媒体⽂件，都是通过其它⽅法，例如 activex插件或者 flash。
⽀持后，页⾯可以在web服务器上放置视频⽂件，浏览器根据⽀持的视频格式请求相应的视频⽂件。
规范
:
If the element has a  attribute: zero or more  elements, then , but with no  descendants.
If the element does not have a  attribute: zero or more  elements, then zero or more  elements, then , but with
no  descendants.
:
- Address of the resource
- How the element handles crossorigin requests
- Poster frame to show prior to video playback
- Hints how much buffering the  will likely need
- Hint that the  can be started automatically when the page is loaded
- Groups  together with an implicit
- Whether to loop the
html如何下载
- Whether to mute the  by default
- Show user agent controls
- Horizontal dimension
- Vertical dimension
video如何下载的？
HTML5 video does not work like streaming technologies or Flash. So how does the browser manage to play a long video without downloading the whole file before playing it? Part of the trick is that the video is encoded in such a way that the
metadata is at the top of the file. This means once the first chunk of data is returned it is enough to determine whether the browser can play it all the way through. If you are encoding video for use with the video element, you will want to choose the Web optimized option in the encoding software. (See the section on video encoding above.)
不⽀持类似流媒体和flash的功能。
The real trick though is how Web servers allow you to only download the a part of a file you request. The browser requests a chunk of the video at a time which allows HTML5 video to give the appearance of streaming. This behavior of mimicking streaming is called progressive download. This also allows fast seeking through the video by clicking the time rail on an
unbuffered portion of the video. In general, requesting just a chunk of a file from a Web server is called a range request or “.”
只要服务器允许请求⽂件的⼀个部分，就可以实现表现上流的效果。可以⽀持拖动进度条，这种下载也成为进度下载（progressive download），
服务器⽀持的这种特性叫 byte serving
You may need to do some configuration on your video server to allow for range requests. You can test this by looking at the response headers for one of your video files. Here we look at the headers for our demo video on the command line.
$ curl -I su.edu/RIS/getting_a_book/getting_a_book.mp4
HTTP/1.1 200 OK
Content-Length: 8343631
Content-Type: video/mp4
Last-Modified: Thu, 20 Dec 2012 19:40:10 GMT
Accept-Ranges: bytes
ETag: "f79b80d2e9decd1:89fd"
Server: Microsoft-IIS/6.0
Access-Control-Allow-Origin: *
X-Powered-By: ASP.NET
Date: Sat, 22 Dec 2012 22:04:23 GMT
使⽤这个命令可以查看是否⽀持 bytes下载。
You will be looking for the “: bytes” header which advertises that the server can accept range requests.
range请求的解释：
You can look at the request headers to see how this works from the client’s perspective. Using the network tab of the browser’s developer tools or an add-on like Firefox’s . Go to  and you’ll see “Range: bytes=0-” as one of the headers the browser sends when making a request for the video. This is the initial request for the first chunk of bytes. A successful response will begin with
a “206 Partial Content” response code. The response will include the “Accept-Ranges: bytes” header to show that range
requests are accepted. The  header (e.g. “Content-Range: bytes 0-3771428/3771429”) shows the range of bytes which were transferred in the current response followed after the slash by the total number of bytes in the file. You’ll also see in the time rail for most players that part of the video timeline has been “buffered” and is available to play. The  header will show the actual number of bytes which were transferred with each request.
live streaming 不⽀持
对于实时流媒体，存在 rtsp 和 rtp协议，但是很遗憾 video标签不⽀持此功能。⽬前只能通过http下载视频。
Abstract
This document describes a protocol for transferring unbounded streams
of multimedia data.  It specifies the data format of the files and
the actions to be taken by the server (sender) and the clients
(receivers) of the streams.  It describes version 7 of this protocol.
I don't think it is possible now to "cheat" the HTML5 browser to encapsulate the live video stream to a ".mp4" file. I believe
HTML5 will consider live video support in a near future. What you can do is just wait. :)
Future of Media on the Web
The simple video element is rather pedestrian these days. There’re so many other interesting things happening with media on the Web.
: Not the same as the <audio> element
: Real Time Communication between browsers
: Sync other content along with other time-based media.
Canvas + Video: Adding a video to a canvas adds all sorts of possibilities for what you can do with video in the browser.
Here’s a demo of an .
: Make it possible to address a fragment of a piece of media using URIs.
Media Groups

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