Polymer 3 and LitElement are separate projects by the same team. LitElement is the successor to Polymer 3. It would have been named Polymer 4, but it's quite di

Polymer 3 and LitElement are separate projects by the same team. LitElement is the successor to Polymer 3. It would have been named Polymer 4, but it's quite different. They're highly compatible because they are both web components-based. There's no "framework" that creates component instances, the browser does that when it sees the custom elements. So each component is in control of its own lifecycle and rendering. You can have a Polymer element use LitElements (or Stencil, or Lightening, etc) or vice-versa.