Michael,
About which kind of projects are you talking? Both F4 (Preview Project) and F12 (Preview SWF in browser) work perfectly in CP8 for normal projects, they generate a temporary SWF. If you want to see the HTML5 output you need F11.
However for responsive projects F12 has no sense whatsoever, because that is only for SWF, F11 works fine. F12, nor F4 will work (because of SWF output), and to preview Project with temporary HTML5 you need to use the Preview button on the BBB. There is indeed no shortcut key for Preview, Project (HTML5). If you hover over the Preview menu options, you'll see a tooltip for most options, but not for Preview, Project. I'm not sure if it is by design or a bug.
Lilybiri