CSS Flexible Box Test: justify-content_center

Test passes if:
1. the rectangle 1, 2, 3 show up in a row in a red rectangle and no gap between them.
2. the rectangle 1, 2, 3 appear in middle of red rectangle.
3. equal amounts of empty space between the left edge of the red rectangle and ractangle 1 and between the right edge of the red rectangle and rectangle 3.
4. the height of the 1, 2, 3 is the same as the height of the red rectangle.

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