Put divs previously under "float atribute" go down
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Let's say i have a section with three divs side by side and the main div (center) occupies 50% of the space, 25% for the ones in the left and the right.

Now, i'm using media queries to display the site in best conditions on kindle portrait (600 x 1024).

Since it's a lower resolution, to make the site look better i thought in puttin the main column appear and in tho bottom of her put the two that was previously at side.

I was able with "float" atribute to put the main div on the left, but it happens that the other two don't go under the main one.

Is that even possible to do? I'm trying but i'm far far away of being able to do what i pretend. 