The death of the line of death
The line of death, as Eric Lawrence explained in a classic blog post, is the idea that an application should separate trustworthy UI from untrusted content. The typical example is in a web browser, where untrustworthy web content appears below the browser toolbar UI. Trustworthy content provided by the web browser must appear either in the browser toolbar, or anchored to it or overlapping it. If this separation is maintained, then untrusted content can’t spoof the trustworthy browser UI to trick or attack the user.
Source: emilymstark.com