什么定义所以网站在移动设备上看起来一样

I'm trying to create a website that looks the same on every browser. I use a font of 13px, and every margin and padding is defined as pixels. Here I created 3 div's so that the leftpart, the middlepart and the rightpart all align vertically. These 3 divs ar wrapped in the "upper"-div. On my desktop (IE and Chrome) it looks good, and aligned. But when I want to view my site on my mobile, the middle and right part are higher then the left. Is there a property I am forgetting to define which has a different default setting in each type of browser?I'm trying to create a website that looks the s