<clipPath>

html,
body,
svg {
  height: 100%;
}

<svg viewBox="0 0 100 100">
  <clipPath id="myClip">
    <!--
      Everything outside the circle will be
      clipped and therefore invisible.
    -->
    <circle cx="40" cy="35" r="35" />
  </clipPath>

  <!-- The original black heart, for reference -->
  <path
    id="heart"
    d="M10,30 A20,20,0,0,1,50,30 A20,20,0,0,1,90,30 Q90,60,50,90 Q10,60,10,30 Z" />

  <!--
    Only the portion of the red heart
    inside the clip circle is visible.
  -->
  <use clip-path="url(#myClip)" href="#heart" fill="red" />
</svg>

/* With a touch of CSS for browsers who *
 * implemented the r Geometry Property. */

@keyframes openYourHeart {
  from {
    r: 0;
  }
  to {
    r: 60px;
  }
}

#myClip circle {
  animation: openYourHeart 15s infinite;
}

A clipping path is conceptually equivalent to a custom viewport for the referencing element. Thus, it affects the rendering of an element, but not the element's inherent geometry. The bounding box of a clipped element (meaning, an element which references a <clipPath> element via a clip-path property, or a child of the referencing element) must remain the same as if it were not clipped.

By default, pointer-events are not dispatched on clipped regions. For example, a circle with a radius of 10 which is clipped to a circle with a radius of 5 will not receive "click" events outside the smaller radius.

clipPathUnits

Defines the coordinate system for the contents of the <clipPath> element. Value type: userSpaceOnUse|objectBoundingBox ; Default value: userSpaceOnUse; Animatable: yes

• Other clipping and masking SVG elements: <mask>
• Some CSS properties: clip-path, pointer-events