viewBox

html,body,svg { height:100% }
svg:not(:root) { display: inline-block; }

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <!--
  with relative unit such as percentage, the visual size
  of the square looks unchanged regardless of the viewBox
  -->
  <rect x="0" y="0" width="100%" height="100%"/>

  <!--
  with a large viewBox the circle looks small
  as it is using user units for the r attribute:
  4 resolved against 100 as set in the viewBox
  -->
  <circle cx="50%" cy="50%" r="4" fill="white"/>
</svg>

<svg viewBox="0 0 10 10" xmlns="http://www.w3.org/2000/svg">
  <!--
  with relative unit such as percentage, the visual size
  of the square looks unchanged regardless of the viewBox`
  -->
  <rect x="0" y="0" width="100%" height="100%"/>

  <!--
  with a small viewBox the circle looks large
  as it is using user units for the r attribute:
  4 resolved against 10 as set in the viewBox
  -->
  <circle cx="50%" cy="50%" r="4" fill="white"/>
</svg>

<svg viewBox="-5 -5 10 10" xmlns="http://www.w3.org/2000/svg">
  <!--
  The point of coordinate 0,0 is now in the center of the viewport,
  and 100% is still resolve to a width or height of 10 user units so
  the rectangle looks shifted to the bottom/right corner of the viewport
  -->
  <rect x="0" y="0" width="100%" height="100%"/>

  <!--
  With the point of coordinate 0,0 in the center of the viewport the
  value 50% is resolve to 5 which means the center of the circle is
  in the bottom/right corner of the viewport.
  -->
  <circle cx="50%" cy="50%" r="4" fill="white"/>
</svg>