Unity WebGL Tutorial

Unity Engine Settings
In player settings once WebGL platform chosen:
Company Name must be changed
Version must be changed to at least 1.0
Default Canvas Width = 960
Default Canvas Height = 540
Default Template

Once Project Built
Zip entire project folder 

Itch.io Settings
Kind of project must be set to HTML
Upload zip folder
Tick 'this file will be played in the browser'
Viewport dimensions must be set to 960 x 600