html,body,svg { height:100% }

by Vince

HTML

<!-- Learn about this code on MDN: https://developer.mozilla.org/en-US/docs/Web/SVG/Element/mask -->

<svg viewBox="-10 -10 120 120">
  <mask id="myMask">
    <!-- Everything under a white pixel will be visible -->
    <rect x="0" y="0" width="100" height="100" fill="white" />

    <!-- Everything under a black pixel will be invisible -->
    <path d="M10,35 A20,20,0,0,1,50,35 A20,20,0,0,1,90,35 Q90,65,50,95 Q10,65,10,35 Z" fill="black" />
  </mask>
 
  <polygon points="-10,110 110,110 110,-10" fill="orange" />

  <!-- with this mask applied, we "punch" a heart shape hole into the circle -->
  <circle cx="50" cy="50" r="50" mask="url(#myMask)" />
</svg>

CSS

html,body,svg { height:100% }