The sandbox attribute enables an extra set of restrictions for the content in the iframe.

When the sandbox attribute is present, it will:

treat the content as being from a unique origin
block form submission
block script execution
disable APIs
prevent links from targeting other browsing contexts
prevent content from using plugins (through <embed>, <object>, <applet> etc)
prevent the content to navigate its top-level browsing context
block automatically triggered features (such as automatically playing a video or automatically focusing a form control)

The value of the sandbox attribute can either be just sandbox (allow-all), after which restrictions are applied explicitly, or deny-all, listing a space-separated list of pre-defined values that will remove the restrictions explicitly.

Bad Practice

<iframe></iframe>
<iframe/>
<iframe sandbox="__unknown__"></iframe>
<iframe sandbox="allow-popups allow-popups-to-escape-sandbox allow-pointer-lock allow-same-origin allow-top-navigation"></iframe>

Recommended

<div sandbox="__unknown__" />;
<iframe sandbox="" />;

blurt / blurt

No ignore rules found