Speaking as someone who reads Chinese and Japanese, this is not as annoying a problem as when a font designed for, say, Chinese is used to display Japanese and

Speaking as someone who reads Chinese and Japanese, this is not as annoying a problem as when a font designed for, say, Chinese is used to display Japanese and does not have the native-Japanese character (say, 働). Europeans using a different Latin script than a subset of English will be intimately familiar with what I am talking about. I don't think that Han unification itself is too problematic: every variant is attested to some extent in handwriting in all these societies. Furthermore, the web designer conscientious enough will 1. For applications meant to be displayed in a particular language, use a font for that language, in which case the lang-untagged content always appears right 2. For multilingual content ensure that lang tags and fonts are set up right. If the owner fails to do this, fallback font usage is going to be just as great a problem as variant issues.