To novices and the weak of heart? Sure. I've yet to find a greater source of technical depth regarding JavaScript.
> David Mark's touted his "My Library" as the holy grail of libraries while bashing any others, yet kept it closed source (now abandoned).
It seems that you've turned a blind eye. His code's been available in a multitude of ways (I've used it myself as a learning tool). There's a GitHub repository[0] as well.
> They are as stable as the code using them, just like their server-side counterparts.
The prime difference here is that most browsers do not throw markup errors or halt execution upon a bad tag; they're extremely lenient. Conversely, direct parser access via JavaScript can throw a host of errors and halt execution (as I noted with IE). Fragment builders are hampered because they're either too dynamic for their own good or need to keep a laundry list of valid attributes.
> Would you mind expanding on security?
Apart from the OWASP article I linked earlier, it's more of a statement towards older environments. Browsers today escape input nicely, but that's meaningless for Joe Smith, government bureaucrat on IE 6. I prefer direct node creation patterns via `createElement` et al. for this reason (especially text nodes).