Hi! From what I know there is no alt attribute for the video tag. <video>: The Video Embed element - HTML: HyperText Markup Language | MDN
If you need this from an accessibility perspective, then it is recommended to add captions.
Read more here: <video>: The Video Embed element - HTML: HyperText Markup Language | MDN
And here: Adding captions and subtitles to HTML5 video - Developer guides | MDN
Is this something that you need?
Hope this helps!