Rendering HTML elements to <canvas>

You won't get real HTML rendering to <canvas> per se currently, because canvas context does not have functions to render HTML elements.

There are some emulations:

html2canvas project http://html2canvas.hertzen.com/index.html (basically a HTML renderer attempt built on Javascript + canvas)

HTML to SVG to <canvas> might be possible depending on your use case:

https://github.com/miohtama/Krusovice/blob/master/src/tools/html2svg2canvas.js

Also if you are using Firefox you can hack some extended permissions and then render a DOM window to <canvas>

https://developer.mozilla.org/en-US/docs/HTML/Canvas/Drawing_Graphics_with_Canvas?redirectlocale=en-US&redirectslug=Drawing_Graphics_with_Canvas#Rendering_Web_Content_Into_A_Canvas

Take a look on MDN (archived)

It will render html element using creating SVG images.

For Example: There is <em>I</em> like <span style="color:white; text-shadow:0 0 2px blue;">cheese</span> HTML element. And I want to add it into <canvas id="canvas" style="border:2px solid black;" width="200" height="200"></canvas> Canvas Element.

Here is Javascript Code to add HTML element to canvas.

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');

var data = '<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">' +
  '<foreignObject width="100%" height="100%">' +
  '<div xmlns="http://www.w3.org/1999/xhtml" style="font-size:40px">' +
  '<em>I</em> like <span style="color:white; text-shadow:0 0 2px blue;">cheese</span>' +
  '</div>' +
  '</foreignObject>' +
  '</svg>';

var DOMURL = window.URL || window.webkitURL || window;

var img = new Image();
var svg = new Blob([data], {
  type: 'image/svg+xml;charset=utf-8'
});
var url = DOMURL.createObjectURL(svg);

img.onload = function() {
  ctx.drawImage(img, 0, 0);
  DOMURL.revokeObjectURL(url);
}

img.src = url;

<canvas id="canvas" style="border:2px solid black;" width="200" height="200"></canvas>

Here is code to render arbitrary HTML into a canvas:

function render_html_to_canvas(html, ctx, x, y, width, height) {
    var xml = html_to_xml(html);
    xml = xml.replace(/\#/g, '%23');
    var data = "data:image/svg+xml;charset=utf-8,"+'<svg xmlns="http://www.w3.org/2000/svg" width="'+width+'" height="'+height+'">' +
                        '<foreignObject width="100%" height="100%">' +
                        xml+
                        '</foreignObject>' +
                        '</svg>';

    var img = new Image();
    img.onload = function () {
        ctx.drawImage(img, x, y);
    }
    img.src = data;
}

function html_to_xml(html) {
    var doc = document.implementation.createHTMLDocument('');
    doc.write(html);

    // You must manually set the xmlns if you intend to immediately serialize     
    // the HTML document to a string as opposed to appending it to a
    // <foreignObject> in the DOM
    doc.documentElement.setAttribute('xmlns', doc.documentElement.namespaceURI);

    // Get well-formed markup
    html = (new XMLSerializer).serializeToString(doc.body);
    return html;
}

example:

const ctx = document.querySelector('canvas').getContext('2d');
const html = `
<p>this
<p>is <span style="color:red; font-weight: bold;">not</span>
<p><i>xml</i>!
<p><img src="data:image/png;base64,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">`;
render_html_to_canvas(html, ctx, 0, 0, 300, 150);


function render_html_to_canvas(html, ctx, x, y, width, height) {
  var data = "data:image/svg+xml;charset=utf-8," + '<svg xmlns="http://www.w3.org/2000/svg" width="' + width + '" height="' + height + '">' +
    '<foreignObject width="100%" height="100%">' +
    html_to_xml(html) +
    '</foreignObject>' +
    '</svg>';

  var img = new Image();
  img.onload = function() {
    ctx.drawImage(img, x, y);
  }
  img.src = data;
}

function html_to_xml(html) {
  var doc = document.implementation.createHTMLDocument('');
  doc.write(html);

  // You must manually set the xmlns if you intend to immediately serialize     
  // the HTML document to a string as opposed to appending it to a
  // <foreignObject> in the DOM
  doc.documentElement.setAttribute('xmlns', doc.documentElement.namespaceURI);

  // Get well-formed markup
  html = (new XMLSerializer).serializeToString(doc.body);
  return html;
}

<canvas></canvas>