Adz - with version 2.5 and later, you can "preview" while you work on a layout. This is possible because the Settings Window is separate from the Game Window, so you can edit the layout in the Settings Window while the Game Window displays the layout.
You can lock the Game Window on the particular screen you are working on in one of two ways: press Ctrl+L when the screen you want to work on is displayed in the Game Window, or use the numbers keys 1-9 to select a particular screen and automatically engage the screen lock.
The Seating Chart, Player Rankings, Blinds Schedule, and Player Movements Screens are all "built-in" screens. You can modify properties of them, but not the overall layout of them. You display them by visiting them directly (using the context menu, or hotkeys F3 through F6), or by including them in your layout's Screen Sets. You can modify their properties by using the context menu when the particular built-in screen is displayed, or on the Layout tab by pressing the Other Properties button.
The cells themselves contain HTML, yes, but the overall structure of the screen is defined by how you arrange the cells using rows and columns on the Layout tab. You cannot define the entire page using your own HTML because frankly it would cause the rendering engine in Internet Explorer to slow to a crawl as the page is updated.
Or, actually you can if you want to give it a try. Just create a single cell, and insert it as the only cell on the screen inside a single Row or Column. Insert all of the HTML you want in there, including as many layout tokens as you need. Every time something on the screen changes (and this will be once per second if you have the game clock or the time of day displayed), the entire screen will have to be re-parsed and re-rendered. It will not be zippy.
I suggest checking out the user manual included with the software. It should have most everything you need to know about layouts.