Currently HtmlUnit only adds 'Origin' header for true CORS requests.

https://stackoverflow.com/questions/15512331/chrome-adding-origin-header-to-same-origin-request

Per the link above, "Chrome and Safari include an Origin header on same-origin POST/PUT/DELETE requests (same-origin GET requests will not have an Origin header)". I checked the latest version of Firefox, it also adds Origin header for non-GET requests.

I think HtmlUnit should better add it as well.