Poster image is first instance on video to impress & engage user to play video. By keeping poster image clear & from video will make user interested to click on play button.
To have HTML5 Video on page is good and to have it with Closed Caption is even better, as it will make video accessible for physically challenged as well.
With HTML5 Video Tag it is very easy and simple to include video in page. Unlike earlier we had to include Flash object and to support that we need to add Flash Player Plugin in browsers.