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<style>
  svg {
    border: 1px solid red;
  }
</style>
</head>

<body>
  <svg id="svg">
    <g id="container">
      <rect width="200" height="300" x="0" y="0" fill="red" />
      <circle cx="0" cy="0" r="140" fill="blue" stroke="green" stroke-width="10"/>
     </g>
  </svg>

const svg = document.getElementById("svg");
const container = document.getElementById("container");
const bBox = container.getBBox();

svg.setAttribute("width", bBox.width);
svg.setAttribute("height", bBox.height);
svg.setAttribute("viewBox", `${bBox.x} ${bBox.y} ${bBox.width} ${bBox.height}`);

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