CSS ONLY动画绘制圆，边框半径和透明背景

I am trying to draw a circle with border-radius, and animate it. I can do this, but what i can't do is overlay elements and set the circles background to transparent, without unhiding the mask. I am not able to make it transparent over elements because the mask needs to be applied to hide the left half of the circle as it rotates to mimic the draw effect.I am trying to draw a circle with border-radius