Best way to build pages, or page equivalent
I would suggest checking out Panels and CSS Injector.

Panels does most of what you are looking for; it presents a user interface for laying out varied content on a page without having to dig into HTML. It's not quite a pure WYSIWYG interface (theme styling might not apply to the Panels UI depending on your set-up), but it is as close as you can get with an out-of-the-box solution in Drupal.

CSS Injector will allow your designers to write CSS specifically for one Panels configuration through the UI, without having to touch the theme files.