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I’m pretty sure the concept of a hidden file was an unintended consequence. It was certainly a mistake.
Coincidentally, it’s been said in a comment of the article What No One Told You About Z-Index, that early CSS design decisions is, in fact, driven by how Netscape’s layout logic in the Mozilla code base had worked at the time.
I wouldn’t know if the statement about CSS history is true or not. However, it’s true that many of the cornerstones we have on web development, like some of the designs of the DOM, CSS, HTML, are unintended consequences, amplified by path dependence.
Mozilla, just like Netscape, or Bell Labs, does not
float in the haven, where the developers have infinite time and knowledge to foresee everything. We have our partners and schedules to answer to, too. However, I do feel that working in Mozilla as a web developer is a privileged role, that we should, to the best of my knowledge, avoid unintended consequences, like what Rob states vividly above.