Markdown/Textile etc is no solution. Client’s will learn how it renders and just use it as crappy hard-to-learn WYSIWYG.

There’s a few lessons to learn when building content editors and CMS’s

1. Some content should be WYSIWYG and some shouldn’t. Some data is structured and some isn’t. Sacrificing yourself on the high-altar of ‘semantic markup’ is silly if the benefits are marginal.

2. People don’t add metadata unless there’s a tangible benefit. (semantic markup is essentially metadata). Read http://www.well.com/~doctorow/metacrap.htm

3. There is a real benefit to providing WYSIWYG. All else being equal then WYSIWYG is always preferable (note that ‘all things being equal’ is the key part of the preceding sentence). I’m a coder and I prefer WYSIWYG if there is no downside.

Just make sure you turn off as much formatting as possible. Text color? No. Font size? No. Configure a few custom “as semantic as you can manage” styles and allow users those in addition to strong, em, ul etc. Need a couple of non-semantic styles? I won’t tell and no kittens will die. And ignore the purists unless they give you a decent argument based on real-world cost vs benefit (and “real-world” doesn’t mean whispering ‘accessibility’ like a magic spell. It means testing in an actual screen-reader)