Polymer just makes it easier to create Custom Elements. It mostly adds templating and data binding, which aren't built-in concepts in the Web Components specs.

Polymer just makes it easier to create Custom Elements. It mostly adds templating and data binding, which aren't built-in concepts in the Web Components specs. Every Polymer element is a custom elements, and they can freely mix with other custom elements. Custom Elements and Shadow DOM being implemented native in Safari means that Polymer is just much smaller and faster on Safari.