Open a Japanese sentence in a narrow column and watch where the browser breaks it. It will happily split 特定商取引法 into 特定商取引 / 法, or push a 。 to the start of the next line. Japanese has no spaces, so the default line-breaker treats almost every character boundary as fair game. To a Japanese reader that looks broken in the same way impor / tant would look broken to you.
Most sites ship exactly that. It is the kind of thing you only notice if you read the page in Japanese, which is most of the point of this whole site.
The rule we actually want
Japanese wraps at phrase boundaries — 文節, roughly a content word plus its trailing particles. It also follows 禁則: a closing bracket or a 。 never starts a line, an opening bracket never ends one. Those two together are what "set correctly" means.
CSS gives you half of it for free:
