Doing some more testing.
I added this meta tag to a Strict document:
<meta http-equiv="X-UA-Compatible" content="IE=8"/>
The user-agent string still reported IE7, but the new CSS feature outline worked. If I remove the meta tag the outline property doesn't render.
And just to make sure, I used document.documentMode to get the render mode, with the meta tag it reports IE8, without IE7.
So this is another nail in the coffin for the user-agent string <- mostly pointless and highly unreliable.
I attached the test code I used. Extract it to your plugins menu and type this into the Console to open up a webdialog:
dlg = UI;;WebDialog.new("Test")
dlg.set_url Sketchup.find_support_file "test.html", "Plugins/"
dlg.show
It will first report IE7 mode. But remove the comments around the META tag and see how the blue outline now renders and the JS reports IE8 mode.
You can also see the different behaviour from regular IE windows.
test.zip