I noticed this because I'm used to closing the external viewer with the Escape key: this does not work when viewing HTML files. In fact nothing works: alt+F does not open the File menu, despite the underlined F, and it even bips at me. I wanted to test another key, so I tried ctrl+N (View > Menu Bar) and... it opens the document in Internet Explorer! Clearly the keyboard is hijacked by IE.
I tried .rtf to test another source, and there, the keyboard works as expected (except Page Up/Down) as long as you do not click inside. If you do, then Page Up/Down work, but the rest does not.
That’s by design. We have no way to know which keys those viewers want for themselves, and indeed something like a web browser needs every key on the keyboard for some websites.
That's because the external viewer window is larger, and also because it stays there until I close it, not following the selection in the lister. And I can open more than one, and see them side by side, for instance.
That's why I was proposing an option: by default all keys would be passed to the underlying app, except the ones designated by the user. That would be feasible, right?