Is it necessary to write both mp4 and ogg. Can't we can have just one??

Answer 1

You can write just one, but I reccomend using both incase the browser of the user doesnt support .mp4 or .ogg

Answer 2

Due to IE

Answer 3

you can, but your browser might not support one of them, so it is recommended

Answer 4

Your code must be able to compatible with all browsers. If not, then you must specify what browsers are supported for your application.