Seeking to a value less than 'initialTime' seeks to 'initialTime'.

http://www.w3.org/TR/html5/video.html#dom-media-seeking

When the user agent is required to seek to a particular new playback 
position in the media resource, it means that the user agent must run 
the following steps: 

[...]

3. If the new playback position is less than the earliest possible 
position, let it be that position instead.