Having described Watermarking PDF documents with Stingray and Java and Watermarking Images with Java Extensions, this article describes the much simpler task of adding a watermark to an HTML document.
Stingray's TrafficScript language is fully capable of managing text content, so there's no need to revert to a more complex Java Extension to modify a web page:
This rule has the following effect...:
Of course, you can easily change the watermark text:
... perhaps to add more debugging or instrumentation to the page.
The CSS style for the watermark is based on this article, and other conversations on stackoverflow; you'll probably need to adapt it to get precisely the effect that you want.
This rule uses a simple technique to append text to an HTML document (see the Tracking user activity with Google Analytics article for another example). You could use it to perform other page transforms, such as the common attempt to apply copy-protection by putting a full-size transparent layer over the entire HTML document.