Here's what I'm currently doing.
For HTML, I use Firebug Add-on for Firefox with the Validator extension.
Launch Firebug by clicking the bug icon.
In the console at the bottom of the page, Click Validator and Start Validation.
To Validate CSS, I like the Web Developer toolbar add-on.
Web Developer toolbar puts handy tools at the top of your browser window. Start by browsing to your web page (NOT the css file), then click the Tools menu to validate CSS. You can also use the Web Developer toolbar for html validation, but it doesn't work for some versions of Firefox.