100% DIV width is not really 100%

Question

When I have a

with width: 100%, it is not really 100%:

testtesttesttesttest

...

#di         


        

Answer 1

Width: 100%, is highly affected by its margin and margin and padding of its parent (body in your case). SO, reset them first

Something like

body {
    margin: 0;
    padding: 0;
}
#div {
  margin: 0;
  width: 100%;
  background-color: red;
}

DEMO