make website adapt to all display size - HTML

[sewrobb]

I'm writing a website with HTML and CSS. My problem is that when I for example run my website in my browser, I adjust all the margins with percent. But then when I run my browser in fullscreen or if I adjust the size of the window the websites different parts fall apart and doesn't fit together as they should. Why doesn't the percent unit fix that problem since it's relative to the size of the window? CSS:\[code\]#aboutMeDiv{ background-image: url('noisyBlue.png'); position: absolute; width: 100%; height: 118px; margin-top: 13.6em; margin-left: -0.7%; opacity: 0.5; transition: opacity 0.3s;}\[/code\]How can I make it "the same way" even if the window changes?Thanks!