I think you may be confused by the "as long as it is clear" langauge of that FAQ item. The FAQ item isn't saying that you must make it clear what the source form of a work is, but rather it is saying that there should be a naturally obvious source form of the work. The GNU GPL defines "source code" in its definitions:
The "source code" for a work means the preferred form of the work for making modifications to it.
You are free to clarify what you believe to be the source form of a work, but you might be incorrect. If you said "These compiled
*.o object files are the source form of my executable, since they can be linked to form my executable" that would be wrong -- the source form of your executable is the original source written in a human-readable programming language. The object files are not the preferred from for making modifications.
In your webpage case, since raw HTML is the preferred form for making modifications to a webpage, the webpage is its own source. (This is similarly true for most software written in interpreted languages.) If a downstream distributor obfuscates the readability of the page's HTML (e.g., for compression) then the GPL's source code requirement requires the distributor to make the unobfuscated version of the HTML available to everyone who receives that obfuscated version.
If you are thinking about a complete web application with server-side code, you must understand that the GPL does not place requirements on works that are not distributed. If users receive a GPL-licensed webpage in HTML, and that webpage happens to interact with a network service, there is no GPL requirement to share the code of the network service. (The Affero GPL (AGPL) is a license that does impose such a requirement.)
If you want to apply a different meaning for "source code" than the one that the GPL already supplies, you can make a modified version of the GPL, though you should not do this without a very good reason (since it will be likely incompatible with the GPL) and you should seek the advice of a lawyer (since otherwise you may draft a license that has unintentional effects or unenforceable requirements). You could also create a GPL exception to the effect of "When you distribute this work in object form, instead of sharing the source code (e.g., preferred form for modifications) as normally required in section 6, you may instead share [something else]." I am unsure why anyone would want to do this, since such an exception undermines the primary distinguishing feature of the GPL and strongly suggests you really are not really interested in using the GPL at all.