But you can enforce it in many ways; for example you can set `\emergencystretch=\maxdimen` (or any reasonably large value) and you'll never have overfull boxes (what you called margin overruns), unless of course you have a single unbreakable "word" longer than a line by itself. (On the TeX.SE website here are a couple of answers of mine about related stuff and alternative approaches: https://tex.stackexchange.com/a/422652 https://tex.stackexchange.com/a/401315 though the canonical question with good answers is probably this: https://tex.stackexchange.com/q/50830 .)

So this complaint of "I can't enforce that there are no margin overruns" is (to use the same word) actually something like "If I don't take any steps to avoid overruns, and moreover ignore the warnings that are specifically about overruns, then there are overruns". This complaint is not specific to you and is surprisingly common actually, which brings me to my main point, which is that the majority of people seem to use TeX/LaTeX without ever having read the manual. Some of them even joke about "incomprehensible" messages like "overfull hbox". But the whole point of TeX, what TeX is at its core, is this breaking-paragraphs-into-lines (in my view, the rest of TeX is just a large wrapper around this basic function of setting boxes).

The preface of the TeXbook describes it as "TeX, a new typesetting system intended for the creation of beautiful books", and for its initial design goals, it does make sense that it chooses by default to declare defeat in impossible situations and overflow margins with a warning to the user so that they can take appropriate action, instead of silently producing an "ugly" paragraph with lines subtly looser than whatever tolerance settings the user has specified. What is remarkable here is that a lot of people are using TeX who do not care about meticulous typesetting of "beautiful books" at all, and will blithely ignore warnings related to such matters.

The lesson may be that such a state of affairs is inevitable, so perhaps a better default for similar software today would be to produce loose lines to fit in the margin, so that results are good even for the users who don't read the manual, or even the warnings, or even proofread the final result.

> What is remarkable here is that a lot of people are using TeX who do not care about meticulous typesetting of "beautiful books" at all, and will blithely ignore warnings related to such matters.

Is this really remarkable? It's the standard tool for writing papers in physics. Get result, put into LaTeX, put on arxiv, repeat. The fact that these people have to constantly Google for obscure LaTeX incantations (faster than TeXbook) is not ideal for them. Indeed, it would be better if it were easier to use without reading a 500 page book.

Anyway, thanks for tip. When I googled for this I found only \sloppy, which did not produce reasonable results.

I should have been clear: I don't think people's behaviour is remarkable here (I've done the same, with LaTeX years ago, and with many other tools); what I found remarkable is the history / state of affairs, where a finicky tool primarily concerned with the finer points of typesetting (TeX) became the standard tool for dashing off papers (thanks to Lamport and LaTeX I guess) even for people not concerned so much with typesetting. It's as if, say, a samurai sword had become the standard tool for chopping vegetables in the kitchen, and everyone was complaining about the awkward shape, the unnecessary sheath, etc. It's neither the tool's fault nor the users', but… something has gone wrong somewhere.