The library was carefullly tested with IE 6+ on Windows, Firefox 2.0+, Safari 3+. It should works under IE 5.5, Firefox 1.x and Safari 2.x as well, but it wasn't extensively tested with these older versions
All the code was written with XHTML in mind as it takes advantage of the DOM and leverages XML Namespaces. Still, there is nothing that inherently prevents you from using it inside an HTML page. Of course, the resulting tag soup will be invalid both from an HTML and an XML perspective.
Most of the times they will, sometimes they won't, you can't count on it. If you are concerned about this issue, you can display error messages next to their relevant field.
JavaScript programming knowledge isn't required in order to use the Validator. Some decent understanding of JavaScript can indeed help you out whenever you need to extend the functionality or troubleshoot something.
You can use images or <button> tags as submit buttons, but unlike standard form buttons, the TMT Validator will not disable them after successful validation.
The TMT Validator is free for download, you can freely use it for building websites, web applications or whatever you want, but it's not Open Source, since you can't redistribute it without the proper permission.
The TMT Validator was designed out of the box to be as future-proof as possible. That's why it works even if the page is served using an XML mime-type. However, before going down this route, you should carefully consider all the implications as it requires strong coding discipline. In other words, unless you really know what you are doing, keep serving your XHTML pages as plain text.