It may have to be with your server config – hosting. Is it Windows Hosting?
Look for the file web.config in your root directory via FTP and change it to something different like web.config.disabled
[tab name="HTML"]
<h3>HTML in W3</h3>
<iframe id="I1" style="width: 100%;" name="I2" src="http://www.w3schools.com/html/DEFAULT.asp" height="700" width="320"></iframe>
More text[/tab]
