Question

So, i have some block, and this block must contains two divs, first div must be at left(attached to block), second at right(attached to block), and this two divs must coverage all block size.

Answer 1

<div id="block" style="width:800px">
   <div id="left" style="float:left;width:50%;"> left </div>
   <div id="right" style="float:right;width:50%;"> right</div>
</div>

Both divs have a width half of the parent’s div.

But you have to be careful with borders as the width defines the width of the content (i.e. without borders). So if you use borders, the right box will be shown below the left, but still on the right side.