How Do You Pitch Semantics?

As you may know already, Dave Shea posted a great markup guide designed as reference for clients who are editing their site and adding content, linked from his article Who Cares about Semantics Anyway? The ensuing conversation in the comments section is definitely worth a read.

The main question is, how do you relate the worth of standards-driven code to a client, enough so that he/she will use that code when updating their site instead of <br />, <b> and <i> tags? Sure, you can promote web standards when (if) you design their site in the first place, and that may or may not be difficult. But when the rubber meets the road and the client is making changes on a page, will they apply the semantically correct solution, or just use whatever seems most convenient.

Usually a client will need a strong motivation, something that increases the value to them of using web standards versus hacked-up code. It's been said that clean semantic code will improve a site's search engine ranking, but this is questionable.

Accessibility, file size, future compatibility - all of these may convince a client, but it's likely they won't. So if the search engine card is out, how do you make a successful argument for the Semantic Web?