That's why many define a "default font stack" - basically a default for each device:

  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen, Ubuntu, Cantarell, "Open Sans", "Helvetica Neue", sans-serif;

On your list, Ubuntu font does not cover Hangul (>50 million people) at all for example. You are blessed to write in writing systems with good default fonts.

Not all of the 50 million people use Linux, so they should be covered with the previous fonts. But i'm curious - if some character cannot be displayed, will the browser try the next (or fallback) one automatically? I've been using this list for quite some time, but maybe it's time to revise it.

Oxygen isn't default on Plasma for quite a long time already. It's Noto.

It is fortunate that you have a lot of fonts to fill with.

Oh that's interesting. Do you have examples? That sounds like a bug in the user agent to me.

You have essentially four cases for Korean fonts:

- Gulim and Dotum, two sans-serif typefaces introduced with Windows 95 (!!!!!!) [1]. They have hand-fitted bitmaps (which are not that bad for small pixelated displays) and awfully hinted vectors. Their monospaced variants (Gulimche & Dotumche) still remain as the only portable choice for monospaced Korean fonts.

- Malgun Gothic, a sans-serif typeface introduced with Windows XP. Yeah this got much better! Except that it is not well hinted and thus eye-jarring for smaller texts (hopefully this concern will be obsoleted in the future). Also in my opinion it is slightly unbalanced than other alternatives.

- Noto & Source Han Sans CJK, a sans-serif typeface shipped with Android. This is even better than Malgun Gothic, except that it does not ship with Windows (hahaha). Its monospaced version is also very good, except that it does not ship with Windows and Android (you read right).

- And of course Apple Korean fonts. In the past they shipped AppleGothic which is slightly better than Malgun Gothic, and nowadays they ship Apple SD Gothic Pro which pretty much matches well with Noto Sans CJK.

Thus you go with Noto Sans CJK, Apple SD Gothic Pro, Malgun Gothic, and (hopefully never) Gulim or Dotum. And you know what? There is essentially a single different Korean font acceptable for each platform and their metrics drastically vary (Malgun Gothic is the biggest outlier IMO). And note that serif fonts are completely absent in this list---there is really the single most portable serif Korean font there, namely Batang, in the same page with Gulim and Dotum. If you want serif fonts in your web page you are simply doomed.

There are numerous attempts to improve Korean web typography. Spoqa Han Sans [1], an extremely subsetted version of Noto Sans CJK, is a good example. And it still weighs 441KB---at such size it will be a full-featured font for English but for Korean it is barely comfortable. And I don't know what can be done with this.

[1] Their prototypes were much older, back to Windows 3.1.

[2] https://spoqa.github.io/spoqa-han-sans/en-US/