No problems here with that webpage.

• https://www.omniglot.com/writing/korean.htm

It use Unicode (utf-8) and there shouldn't be a problem.

It looks that in your case there is a problem with the font that is used.

You can compare the actual font that is used in both cases.

You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected.

• https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Examine_and_edit_HTML

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Font tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website.

• https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Edit_fonts

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Font tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website.

• https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Edit_fonts

Hi cor-el, thanks for your help!

I have checked the actually font that used for Korean characters in this page in my case, it is Heiti SC (the related CSS selector is just "sans-serif"), which is a Chinese font that clearly shouldn't be used for this. I have checked other webpages with Korean and it seems have the same issue.

However, Heiti SC is not configered as default Korean font, or Chinese font in my browser (all font settings are remain default), so I still can't figure out why this font is used to diplay Korean.

Some prefs that you can try to modify on the about:config page.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

layout.css.font-visibility.level = 3 Visibility level of font families available to CSS font-matching:

1 - only base system fonts
2 - also fonts from optional language packs
3 - also user-installed fonts

layout.css.font-variations.enabled = true

• https://searchfox.org/mozilla-release/source/modules/libpref/init/StaticPrefList.yaml#line-6693

The "layout.css.font-variations.enabled" has been "true" already. I have changed the "layout.css.font-visibility.level" to 1 but same issue still occure, I believe Heiti SC is an font that came with the system.