You can identify a request for HTML5 when the value of the Video.allowed_video_formats field is set to VIDEO_MP4 = 1. See the Real-time Bidding Proto for information about the video object in bid requests.
To respond with HTML5, video creatives served in your VAST tag must include a <MediaFile> with an MP4.
See sample VAST tags.
Video creatives which are not HTML5-compliant are filtered from the auction for any bid request originating from an HTML5 video player. All video buyers are certified to serve HTML5 creatives.