CSS Ellipse Segment Math - Click in the gray area!

Meta-Method to solve this seemingly hard problem:
usage of a Coordinate System Sequence + basic math.
The red ellipse in CS0 is a unit circle in CS5 :)

width (100..300):

height (100..300):

CS0 (canvas)
CS1 (blue border)
CS2 (blue background)
CS3 (green background)
CS4 (green border)
CS5 (red ellipse)
eccentricity in CS4 (+):

DIV (ellipse segment)

linewidth (1..40):

overflow:hidden clip-path:polygon

polygon segments (2..12):