Video and audio: playing media in the browser - HTML5 in Action - page 347

Figure 8.5. An MP4 video in Firefox, where video format or MIME type isn't supported
The issue illustrated in figure 8.5 is that neither Firefox nor Opera supports the MP4 video
format even though they support the <video> element itself. [ * ] But the <video> and
<audio> elements provide a workaround for this issue: It's possible to specify multiple
media files by using the <source> element.
* Recent versions of Firefox will play MP4 videos on Windows using the support available in the OS.
8.3. Specifying multiple formats with the <source> element
Each <video> element can have multiple <source> elements as children. Each
<source> specifies a video, and the browser tries each one in turn and uses the first video
format it can support. Figure 8.6 shows the same video player in Firefox we showed you
earlier after <source> elements have been added, instead of using the src attribute.