I have a Asp.net web application running with the following config setting.
<xhtmlConformance mode="Legacy"/>
This limits use of AJAX and compatibility with multiple browser.
If my understanding is correct, the HTML code of the aspx pages need to be fixed to comply with XHTML 1.0 Transitional.
There are alot of HTML pages, ~1000, is there a tool that could speed up this process?
Are the pages currently in HTML or XHTML? If they are in HTML, leave them. There is no benefit to using XHTML.
Tidy is probably the best there is, but I've never been entirely happy with it, and if the code is that bad it is usually a sign that it needs rewriting anyway.
HTML 5 supports the use of XML syntax so moving from legacy HTML to XHTML is going to have some long term advantages since you pages will be syntactically correct which could allow for an easier move to HTML 5 or XHTML 2 when they are completed.
As noted by David Dorward in his answer, HTML Tidy might be useful if you have fairly constant HTML markup, but given the number of pages I would be surprised if that was the case. If you do a Google search for "legacy html to xhtml tool" some other tools turnup that might be worth looking into.
Another thing to take into account is how the pages are written. Most legacy HTML pages don't have a clear separation between markup and styles so you are likely going to have to pull styling out for CSS files that automatic tools can be hit or miss with. As such, you might be looking at a situation where the best thing to do is sit down with the site and run the pages though a validator such as the W3C Markup Validation Service or use a browser plug-in to do the testing while you are browsing through the site. The Html Validator for Firefox is pretty good and this is generally how I test web applications. The downside to this that if there is any HTML that is being generated in the code-behind pages, you may not encounter it during testing.
The DMS Software Reengineering Toolkit is a tool for automating mass change to large source code bases.
DMS uses language-precise parsers to read source code, and langauge-precise analyzers to determine meaning or problems. One can use Source to Source program transformations to carry out changes of the form, "if you see this, replace it by that" stated in terms of language surface syntax. The patterns are driven by the language grammar and are applied to ASTs generated by DMS parsers.
DMS has language front ends for a variety of languages, including HMTL and XHTML.
DMS has been used on "dirty" HTML (even that has a precise definition!) to refactor it. It might be a pretty good match for your fix-1000-pages problem.