Then so is something in HTML that doesn’t use CSS3 or JavaScript, for the same reason that a C program that contains no loops is still a program.
HTML is not "C without loops". It is, by definition, a subset of the string type. All HTML is a string, but not all strings are HTML.
The following is valid C, but I would absolutely argue that it isn't a program:
char str[] = "Geeks";
Still, that C example is more of a program than any HTML is because it contains an instruction. It manipulates memory.
HTML is literally just a file format. It is not any more of a program than a PNG file is.
Then so is something in HTML that doesn’t use CSS3 or JavaScript, for the same reason that a C program that contains no loops is still a program.