Other than submit individual web pages for verification on the W3C site, are there any standalone tools that will do this job.
Ideally this would be a visual studio plugin that could catch errors at design time but one that would just take a wep application url running locally would be good.
Open source suggestions would be preferable
I use the FireFox plugin "HTMLTidy" to do this. When you are testing, you can point Firefox to your dev server (i.e. "http://localhost:52457/...") and it will work as if the site were live.
Also consider setting HTML errors to be raised as compile errors in VS and set your compatibility level/Doctype to XHTML 1.1 Strict to get them all. These settings are under Tools -> Options -> Text Editor -> HTML -> Validation
The W3C Markup Validator is an open source tool, which you can download and use it offline.
On Fedora, do:
yum install w3c-markup-validator
CSE HTML Validator is one I have used in the past.
It is not open source, but there is a free version.