Hi Piet, can you share here the content of your *.addin file?
However, I suspect the problem you have is related to FEATURE_BROWSER_EMULATION issues that make the embedded web browser operate in IE7 Standards mode. (see an old blog of mine on the topic)
If my guess is correct your options are:
add the X-UA-Compatible meta tag to the web page you are trying to view
or…
manually add the registry entry for FEATURE_BROWSER_EMULATION support for the IDE.
or…
Install the DevStart.io addin which applies the FEATURE_BROWSER_EMULATION fix for you.