Project: flex
Task (easy): add divs(class 'header', 'footer', 'main') with arbitrary text in '.root' element. '.root' occupies total viewport and children elements together occupy total '.root' space. header is always fixed at the top of '.root'; footer is always fixed at the bottom of '.root'; main occupies the remaining '.root' space. Use css flexbox only, NO css grid, float and position, NO js.