Embed HTML and JavaScript 

The ability to embed HTML and JavaScript  in New Sites is now available

To get started, select Embed from the Insert menu and choose Embed Code.

If inserting the code doesn't work for you try the following:

• Clear your cookies and cache

• Try resetting  browser settings

• Try using in an incognito window

• Try using Chrome with all extensions and plugins disabled

• Try a different web browser

• Try clearing your DNS cache

• Make sure that any links go to secure (https://) URLs

• Any code like target="_top" must be changed to target="_blank"

If it works in the incognito mode this indicates that you have an extension that is causing the problem. It may be an ad blocker or another extension. The Grammarly extension has often been the culprit.

Read more here

Steegle's Instructions

Also Insert external websites to new Google Sites using iframes and create gadgets using Google Apps Scripts

Examples