So, The issue here is that HTML 5 Video tag does not work when hosted in SharePoint library.
Issue resolution is to enable Blob cache and make sure mp4 is listed in path variable
- Open your web.config.
- Search for “blobcache”.
- Set “enable” to True.
- Set “maxsize” to 5. (Optional. In this step 5 means 5 Gb of cache is allowed)
- Ensure that mp4 is listed in “path” value.
Other option is to host your mp4 video outside SharePoint.