CSS (Cascading Stylesheets) does not replace HTML, but it is an alternative to the presentational elements of HTML. You can reduce the file size of a page by using CSS to handle the positioning of items, instead of relying on some sort of nested table construct. Better still, you can reduce the file size (and enhance the flexibility) of an entire website by creating one or more stylesheets that are responsible for controlling the presentation of all of your pages.
There are no WYSIWYG editors that do great CSS; however, there are one or two that don't do a bad job. If you have money to spend, I recommend Macromedia's Dreamweaver software. It may be a bit expensive, but it does a better job than any other WYSIWYG software I know of. If you want something for free, I recommend Nvu, which is based on the original Mozilla Composer.
Remember: There is no substitute for hand coding. HTML and CSS are really quite easy to learn, and humans can do a far better job of web design than a WYSIWYG editor can.
Keystone Websites (business site) | si-blog (personal site with affiliate links)