use html5's new attribute srcdoc (srcdoc-polyfill) Docs
<iframe 
    srcdoc="<html>
              <body>Hello, <b>world</b>.</body>
           </html>">
</iframe>
Browser support  - Tested in the following browsers:
Microsoft Internet Explorer
6, 7, 8, 9, 10, 11
Microsoft Edge
13, 14
Safari
4, 5.0, 5.1 ,6, 6.2, 7.1, 8, 9.1, 10
Google Chrome
14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24.0.1312.5 (beta), 25.0.1364.5 (dev), 55
Opera
11.1, 11.5, 11.6, 12.10, 12.11 (beta) , 42
Mozilla FireFox
3.0, 3.6, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 (beta), 50
If your HTML contains double quotes, consider escaping its double quotes
<iframe 
    srcdoc="<html>
              <body>"Hello"</body>
           </html>">
</iframe>
...or use Javascript to set the srcdoc attribute dynamically
iframe.srcdoc = '<html>
                  <body>"Hello"</body>
               </html>';
Remember wait for the iframe's onload event  before trying to use Javascript to manipulate the iframe's content (before adding event listeners, etc)