By Noxxxx from https://www.noxxxx.com/?post_type=post&p=1291
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通过 canvas 将 svg 元素生成图片的形式，其中图片的大小取决于 svg 元素的复杂度。

const svgString = new XMLSerializer().serializeToString(document.querySelector('svg'));

const canvas = document.createElement('canvas');
const ctx = canvas.getContext("2d");
const svgElement = document.querySelector('.svgElement');
canvas.width = svgElement.clientWidth;
canvas.height = svgElement.clientHeight;

const DOMURL = self.URL || self.webkitURL || self;
var img = new Image();
var svg = new Blob([svgString], {type: "image/svg+xml;charset=utf-8"});
var url = DOMURL.createObjectURL(svg);
img.onload = function() {
  ctx.drawImage(img, 0, 0);
  const png = canvas.toDataURL("image/png");
  console.log(png) // base64 的 url
  // document.querySelector('#png-container').innerHTML = '<img src="'+png+'"/>';
  DOMURL.revokeObjectURL(png);
};
img.src = url;