Firstly, the file size between these formats is vast, a typical HD 60 second MP4, for example, maybe around 80Mb, this will need to be streamed at around 8Mbs. An HTML5 video would be around 1/10th of this size, so around 8Mb, so you can see the bandwidth saving straight away!
HTML5 video uses vector graphics for text and graphics; this is a highly scalable format, which does not pixelate. You can display a vector graphic billboard size, and still, the curves would be smooth.