The implementation difference in the RadHtmlPlaceholder is that in out-of-browser applications it uses the Microsoft WebBrowser control and in browser it uses an IFrame. As far as I understand the issue is that by default the WebBrowser renders pages in IE7 standards mode. The easiest fix/workaround that I found is to add an "x-ua-compatible" attribute to your page:
You can read more about this attributes here, here
I hope I was able to help you and if you have more questions please feel free to ask.
TRY TELERIK'S NEWEST PRODUCT - EQATEC APPLICATION ANALYTICS for SILVERLIGHT.
Learn what features your users use (or don't use) in your application. Know your audience. Target it better. Develop wisely.
Sign up for Free application insights >>