Requirements for VTT subtitles

The VTT subtitles

To add subtitles to your audio, video or interactive video, copy the contents of your subtitle VTT file into the vtt Subtitle Content text field.

Remember that the first line of text must have the word WEBVTT followed by a blank line and the time format must be 00:00:00:00.000. 

To learn more about the VTT format and to see examples see: https://www.w3.org/TR/webvtt1/

If there is an error in the content of the VTT, the text box will be highlighted in red:

You can use a validator like this one: https://quuz.org/webvtt/,to get more information about the error.

If you do not have a subtitle VTT file, there are many online tools to generate it and you can even do it through Youtube, we recommend this option as it is simple and free.