Defining the natural language of a web page is very important: it allows assistive technologies like screen readers to read the content of a web page out loud using the right accent en pronunciation. In (X)HTML you can define the natural language both for the whole page and particular elements using the lang and xml:lang attributes. As marking a particular fragment of text with a different language isn't really difficult and can be done by the end users, setting the natural language of the whole page dynamically in SharePoint is challenging and must be implemented by developers.
Designing and developing accessible web sites on top of SharePoint 2007 gets more and more attention in the community. But the more developers try to reach the required accessibility or standards compliancy level, the more challenges they face and the more questions pop up. One of such questions is which doctype should be used for standards compliant and accessible web sites.
Each time you request a Site Collection (http://domain/) or a Site (http://domain/foo/) of your Publishing Site you get redirected to the http://domain/Pages/<WelcomePage>.aspx. SharePoint 2007 uses the 302 header (location temporarily moved) for this purpose. Surprisingly even WSS uses the 302 header to redirect a root url to the default.aspx. In comparison ASP.NET uses an internal redirect to render the default page when the root url requested: there is no redirect in this situation.
Accessibility is a broad term and reaches way beyond the standards compliant code only. Accessibility is in my belief a set of features improving the understanding of information presented by an information system. I have to admit though compliant and semantic HTML is a very important factor of accessibility as it hosts the information. As I have recently solved the issue of standards compliant HTML in SharePoint 2007 I have started looking for new challenges and accessibility improving solutions. Almost immediately I have stumbled upon automatically marking up abbreviations in content.