When viewing a website in Devtools with the Responsive Viewport, you can customize the screen dimensions in pixels. Is this accomplished on the browser end according to CSS/JS rules? , or are the screen dimensions sent to the website server, which responds appropriately? If it's the latter, how is this info sent - is it part of the useragent, header, HTML tags, something else?
My goal is to display a website with custom viewport dimensions with something like a UserScript, and without using DevTools or physically resizing the browser window.
