encyclo - 3:41 pm on Feb 21, 2006 (gmt 0)

CSS validation is much more important than HTML validation when it comes to accessibility: in particular using an automated validation tool is vital for spotting errors such as missing or clashing colors/background colors which could seriously hamper readability for users with custom styles enabled.

For HTML validation, as le_gber mentioned, some are more important than others: missing trailing slashes in XHTML are irrelevant, but missing alt attributes or broken, mismatched or ill-formed tags can make your page inaccessible.

Unless you are confident that the specific errors you are introducing will have no impact, then you should always aim for full validation of your pages.

However, it is perfectly to have a 100% valid but totally inaccessible page, as much as it is to have an invalid page which meets most accessibility criteria. Validation is merely another tool which will help you ensure the quality and accessibility of your site.