As a developer who came of age at the dawn of Web 2.0, supporting semantic markup is something that I've always been in favour of. I've written in the past about how HTML doesn't need (much) styling to be perfectly functional. So I'm really enjoying the fact that today (April 9th) is "officially" CSS Naked Day.
If you're reading this on the 9th, then my whole site will be untouched by CSS. If you're reading on any other day, then it's only this page that's unstyled.
Apart from being a fun exercise, there is a serious point to be made here. A lot of people experience the web via nothing more than the markup (be that through assistive tech like screen readers, or just automated delivery mechanisms like web-scrappers and RSS feeds). The way we structure our documents matters.