The most common cause for that type of problem is that different browsers have different default margins and paddings. If you start your style sheet with *{margin:0;padding:0;} this resets them all to zero. After that most browsers will give them the settings your css sheet specifies.

One thing that isn't related to this problem, but your 'site.css' sheet has 3 very weird characters at the start of it. They are called byte order marks and they will sometimes show up on a page produced by html. They won't do that from a css sheet, but they aren't good and probably make it invalid. If you can set your site editor to work without BOM (byte order marks), that will keep those characters from appearing.

If you can't, you want to check every file that you upload after you upload it, and get rid of them.

So a good start would be to replace those first 3 characters with *{margin:0;padding:0;}