Update lib/canvas.js

Co-authored-by: Linus Unnebäck <linus@folkdatorn.se>

Author: jimmywarting

lib/canvas.js

@@ -52,6 +52,8 @@ Canvas.prototype.createJPEGStream = function (options) {
  * @since NodeJS v18.0.0
  */
 Canvas.prototype.convertToBlob = async function (options = {}) {
+  // If the user agent does not support the requested type, then it must create the file using the PNG format.
+  // ref: https://html.spec.whatwg.org/multipage/canvas.html#a-serialisation-of-the-bitmap-as-a-file
   const type = options.type && FORMATS.includes(options.type)
     ? options.type
     : 'image/png'