<svg width="200" height="200" viewBox="0 0 200 200">
<defs>
<mask id="holeMask">
<rect width="200" height="200" fill="white"/>
<circle cx="100" cy="100" r="20" fill="black"/>
</mask>
</defs>
<path d="M0 0l200 200" stroke="red" stroke-width="5"/>
<path
d="
M 50,100
C 50,50 150,50 150,100
C 150,150 50,150 50,100
"
fill="blue"
mask="url(#holeMask)"
/>
</svg>