hubspot sidekick copy - Create Web bug / web beacon / clear gif - and build email tracking app same as Hubspot

PLEASE DO NOT REPLY WITHOUT READING FIRST Some emails include a 1×1 CLEAR GIF image, they’re called web bugs, or web beacons and they track when emails are opened. Hubspot do a superb one check it out here: [login to view URL] Hubspot's email tracker comes with a nice email client widget called sidekick. I would like to clone this application and functionality and add it to my ATS (recruitment system) Please only reply if you can tell me what tech Hubspot is using and know how to copy this. I am technical enough to know if you are wrong thanks