I have been experimenting with Book Designer. Although BD reformats any HTML loaded, the browser seems to display any and all tags entered.Using the HTML Fragment Editor and a text file containing the the tag data, BD can be used as a WYSIWYG.
The attached image shows the setup I've been using. By highlighting the text you want to format and clicking the HTML Fragment Editor, the HTML for the highlighted text is displayed. You can edit the fragment as a <blockquote>, or whatever, click the APPLY icon, and the change immediatly shows up in the editor. I haven't been able to test a lot of tags but those I have tried have worked.
To do a Global Find/Replace, click on Whole Code in the Fragment Editor and when the entire file is displayed click on the Editor Find Icon. If there is a problem in the tag tree, it is highlighted in red and a description is printed at the bottom of the screen. It's no speed demon, but it works.
It's not as good as a split-screen-synced editor/browser but it works. It takes a few moments to setup the screen I use, It works for me.