the left blue circle smoothly animates its stroke-width from time t=1 second to time t=5 seconds, so that the blue color fully fills the left donut shape
the right pink circle smoothly animates its stroke-width from time t=4 seconds to time t=7 seconds, so that the pink color fully fills the right donut shape
both of the stroke-width animations originate from the thin black circle in the middle of each donut shape
