@arspr: As is mentioned in the welcome text on the preview screen, page breaks, page margins and embedded fonts whose font-family names in the css dont match the font-family names inside the actual font will not work in the preview.
And I'm afraid I am never going to agree that preventing all the automatic tools, the live preview and the live css editor (I am currently working on) from working just because you have forgotten to close a tag is a good idea. Imagine you are editing an HTML file and you decide to enter a new paragraph, you type in the <p> and start typing in text, everything will stop working until you type the closing </p>. That is just silly.
And the calibre philosophy has always been to empower people to do things even if they dont have as much technical knowledge as you or I. Having an editor that punishes you in scenarios like the previous one, is simply not compatible with my philosophy.
As it it, if you feel you want to have exact control over the HTML in the file, just never use any of the automated tools, or make sure that all the HTML is always perfectly valid XML before running the automated tools, then nothing will be changed apart from the doctype, the xml declaration and entities. Everything else will be exactly as you typed it.
I am done talking on this subject. As always, if you disagree, you are welcome to your own opinions, just realize that I am just as entitled to mine. And since I am doing all the work, my opinion prevails.