1}Windows 7
2 & 3)History of the browser user-agent string:
http://webaim.org/blog/user-agent-string-history/ [webaim.org]

In the beginning there was NCSA Mosaic, and Mosaic called itself NCSA_Mosaic/2.0 (Windows 3.1)
...
And then Google built Chrome, and Chrome used Webkit, and it was like Safari, and wanted pages built for Safari, and so pretended to be Safari. And thus Chrome used WebKit, and pretended to be Safari, and WebKit pretended to be KHTML, and KHTML pretended to be Gecko, and all browsers pretended to be Mozilla, and Chrome called itself Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/525.13 (KHTML, like Gecko) Chrome/0.2.149.27 Safari/525.13, and the user agent string was a complete mess, and near useless, and everyone pretended to be everyone else, and confusion abounded.

Thank you, phranque. The user with this user-agent reports problems with my website's design. Reported problems (shifted divs) may appear in Internet Explorer 6 but not in modern browsers. He says that websites's design appears destroyed in all modern browsers. My website has been tested in all possible browsers and no problems were found. And noone from several thousands of users has reported problems like this. I wonder what's going on in computer of this user.

have you tried validating the HTML?

yes, there are errors like "element is absolete".

But as I wrote, noone else of thousands has reported this problem. Only 1 user.