That is... a cool trick! I don't know C, so it would have never occurred to me. (Let's be honest; it would have not occurred to me even if I knew C :))

What I like about the Bash + Heredocs + substitutions approach is that it looks and feels declarative to me. Template "expansion" is just in-place substitutions (parameter expansion, shell substitution, process substitution); no string munging needed to "place" or "inject" content in the HTML tree.

Anyway, I spent way too much time figuring that out, and it works well for me, so I'm going to just roll with the sunk costs :D

Thanks for sharing your approach!