actually am having trouble with my site, am using first sction with 100 % and in it a div with 85% and in it there are two divs one flaot left and the othr is float right, i dont know y but on low resolution left div is on left but the right one is under it so
Well if the divs inside the 85% div are a fixed width in px, that could be your problem. Try making them a percentage that adds up to under 100% or equal to. If there are any margins or padding on the left or right of them, you could make those percentages as well and make sure the width, margin, and padding values all add up to under 100% or equal.
You may want to avoid any min-width values as well if you’ve specified any.
Your problem has to do with the 400px. 400px is fine at a large screen size, but once the container gets down to less than 800px wide then it breaks. This is because you have your left column set at 50%, but at less than 800px wide the 400px column takes up more than 50% of the space, meaning there isn’t room for both of them.
You can either convert them both to percentages that add up to 100% or less, or use the flexbox capability to make the right column take up 400px and then the left column to fill the remaining space. Either way, you’re going to have to compromise on one of the two columns.