The real pros used server side includes, where you could drop specially formed comments into your HTML which then be processed by Apache and replaced with the content of another file before being sent on to the client.
Frames lead me to IIS SSI’s which lead me to PHP 3 which kicked off my entire career.
Likewise, I think its under appreciated just how many people got their start in this industry through tinkering with things because they seemed interesting.
i got my start with a copy of Gorillas - a QBasic game - when i was maybe 7 or 8. to play it you had to open the code and execute it. one day i got curious what all of the gibberish text meant, and that was the start of my journey: a damn game of gorillas throwing bananas! now here i am.. 30+ years later and eternally grateful for those pixelated primates. i sometimes wonder what drove me to tinker with that code that day.