Some web players use a 920x516 resolution. This maintains a roughly 16:9 aspect ratio suitable for many online platforms.
Sometimes "516" and "MP4" appear together in specific hobbyist or professional niches: How To Upload Video on Google Drive and Share Link 516 mp4
To change a file (like an MKV) into an MP4, use VLC Media Player by selecting Media > Convert/Save and choosing the Video - H.264 + MP3 (MP4) profile. Some web players use a 920x516 resolution
Upload your video to Google Drive by clicking New > File upload . To let others view or download it, change sharing permissions to Anyone with the link . 516 mp4