As a part of work, I've come across content sniffing, and i understand web apps can be vulnerable to xss because of it. There is another post, regarding content sniffing and xss, but didnt quite answer my question, or maybe i just misread it. content sniffing will read the file to try and determine what type of file it is. If there are malicious html tags, they may be rendered.
So say i go to www.xyz.com, it loads, (using chrome for this example), i right click --> view page source, copy the page source into an html file, save on my desktop, added a xss line within the page. I reload in chrome and page loads with the typical alert box. Now, because the modified html was loaded from my desktop, is this not a xss vulnerability on the website's end?