====== Incrustar HTML en un correo de gmail ======

  * https://www.campaignmonitor.com/css/style-element/style-in-body/
  * https://www.gmass.co/blog/use-custom-html-in-gmail-compose-window/
  * https://quip.com/aO0pAZO9m9SG
  * https://chrome.google.com/webstore/detail/free-html-editor-for-gmai/ioinaaeeacahcmbgfmeaaofhfkijpdeb
  * https://litmus.com/community/discussions/1275-gmail-app-gmail-webmail-and-inbox-metrics
  * https://stackoverflow.com/questions/1033156/how-to-write-ahover-in-inline-css
  * https://codebeautify.org/image-to-base64-converter