WP has a video shortcode you can used to add an .mp4 video to your website. However, I couldn’t find where you could add a splash or poster image, which defeats the purpose of the shortcode (it was tooted as an alternative to plugins.)
Plus, the mp4 would play in Firefox.
So let’s look a pure HTML5 video. Here is a good resource for the html and code behind it, according to the IE Dev Center:
Adding an HTML5 video control to your webpage
there is a poster image example on there as well as the different video formats.
Turns out HTML5 is good at listing multiple formats for video in an orderly [coded] fashion.
Not converting videos.
Here’s a great place to get your hands dirty practicing it:
based on the official W3Schools page: http://www.w3schools.com/html/html5_video.asp