Edit Book is official...
I've just seen the new Font Name warning and I've quickly tested the CSS style attribute test. (The cryptic error text we get is a small drawback, but as you explained, it's out of your reach. In addition, even if possible, spending but little effort in fixing it is probably pointless nevertheless). Again
And now a small suggestion about ToC Viewer. I find really useful and user-friendly the Red/Green icons about "unreachable/reachable" entries you already have in the ToC Editor tool. Could you port it to ToC Viewer? In this way from the Book Editor you could see if you already have troubles in your ToC without needing to open another different tool.
And another suggestion for ToC Editor (and Viewer if possible). By default nested items are collapsed, so you don't know if you have trouble somewhere in your ToC till you expand it completely. Could you create another icon for this situation? I mean:
- Green (modified behaviour): The parent entry AND all its children (no matter in which level) are fine.
- NEW Orange/Yellow. The parent entry is fine BUT you have trouble somewhere in its children.
- Red. The parent entry (or lonely entry) is wrong. (It doesn't matter how its children are).