A div in the form with an id and then using innerHTML to update the content of the div with the error message has been the standard way of producing error messages on forms for the last eight or so years. That's why browsers repurposed alert as it was only necessary for displaying messages in Netscape 4 and earlier browsers and more modern browsers such as Internet Explorer five provided a cleaner way to do it.

That's OK, I agree.
So, maybe, if miniBB team enjoy with this solution they may provide it in future version

We have made alerts the part of miniBB's interface. It's the simplest approach to report user something. It comes in a modal window and can't be just closed, and it doesn't require jQuery or similar libraries, which size is bigger than miniBB's package. It just few bytes of code, so we're happy to live with it. It's truly visible. The approach you mention above (div/innerHTML) is not that visible and can be simply ignored.

Are you coming from the brontosaurus period? Just show me some live page which will work without JavaScript nowadays.

You risk to wait forever. We have no plans to change behaviours of thousands of people just because of two opinions.

Could you please point us to the article or document, which clarifies such kind of standard and proofs it's a "standard way"? I personally never read about that.

Each project is being developed using approaches which fit it best.

Guys, it's very simple to criticize something just because you don't like it. How about offering more practical approach? It's open source. Take it, modify it and re-submit to miniBB project. Then it will be your participance. Until that, you all just sound empty.

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This is pity, and going to end. Firefox so far was the only browser allowing to simply switch JavaScript off in Options, just checking out one box. Now even Firefox maintainers removed this feature, "this change was done to help protect the typical user from accidentally breaking Firefox and the web" - this is from their forums.

You see, regarding Firefox staff, switching JavaScript you break the web.
So how miniBB could be guilty on that. miniBB just uses the simplest approach, and it's JavaScript. And miniBB surely comes for typical users.