one of the first things i did with html was to attempt to use m4 to simplify my personal home page. not only did that not work, it backfired to an incredible degree, but that's because i was using proprietary vendor m4. gnu m4 has a couple of features that make it significantly less error-prone:

1. built-in macro names that require arguments such as `define' are not "expanded" if you don't follow them with parentheses

2. there's an option to require a magic sigil character to invoke macros, such as % or something; this turns out to have been required in the original gpm and its bell labs clone m6. this helps enormously with complicated macros

however, the fact that the output of your macro is always immediately run as new code (as opposed to m6 and gpm, where this was optional on a per-call basis) makes m4 just ridiculously error-prone. i feel like trauma from debugging m4 macros was a major consideration in the design of the weak-ass c preprocessor