This demonstration shows how three divs can be resized using simple CSS, and each also responds to changes in the other divs. The top div is set to resize vertically and the left hand div to resize horizontally.
Resize can be horizontal, vertical or can be set to both.
The button to resize the divs is rather small and easily overlooked.
Drag the corners and observe how the divs respond.
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The HTML code:
There is nothing special about the HTML code. The ability to resize a div is set in the CSS.