Unicode assigns a number to each character. 国 and 國 are different code points. A font is a drawing of those code points. If the file has the right character but the font has no glyph, you see a box. If the file has the wrong bytes, you see mojibake.
This is why conversion and fonts are related but not the same job. OpenCC can change 国 into 國. It cannot install 微软雅黑 or Songti TC on someone else's computer.
One writing system, many fonts
- Simplified-focused fonts may lack uncommon Traditional forms.
- Traditional-focused fonts may lack some Simplified forms.
- A large CJK font (or a pair of SC/TC fonts) is the usual fix.
Operating systems already ship fallback fonts such as Microsoft YaHei, PingFang and Noto CJK. This site uses those system fonts in CSS rather than bundling font files.
Documents
When you convert a Word or PowerPoint file, the characters in the file change. Recipients still need a font that can draw them. If a colleague opens your file and sees boxes, send them the garbled Chinese checklist before you convert again.