> You're now arguing that source code doesn't constitute a program.

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It doesn't. It is the textual representation of a program. As for what the corresponding program is -- for compilers it's simple of course. For intepreters it's a bit more diverse.

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• If the interpreter targets a VM or a compiler infrastructure it would be the IR for that.
• If the interpreter uses the source code directly by parsing a statement and executing the corresponding instruction on its host, that source still isn't a program. Rather, there never is something that could be regarded as complete, assembled program described by the textual input of the interpreter.

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As for whether HTML can be considered a declarative, domain-specific programming language: You might consider it one -- in some restricted sense. In the end the difference between markup language and declarative DSL is whether its use is to annotate documents or describe a computation.

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And that's the deciding factor. Your argument isn't right or wrong. It's simply pointless. If you download a HTML document to view in a browser or plain-text, the language is used as markup -- to annotate a text document. But if you write a rendering engine or something akin that does semantic/syntactic analysis -- then you would treat HTML as declarative DSL.