Make two div elements in html and use the border-radius property to round the corners.
Each corner of the rectangle can be shaped individually. In this example, the top corners are smoothed away with with border-top-left-radius and border-top-right-radius.
The left rectangle which is in red has the property position:absolute and is rotated 45°.
The left rectangle colored in pink is positioned relatively to the red part and is rotated 270°.