Use (or emulate) system font substitution for tabs
Vivaldi (1.7.735.45 x86_64 on Windows 10) does not use system font substitution for non-Western characters:
The screenshot shows three windows on the same system (Windows 10 Simplified Chinese, default UI theme):
- Outlook 2007 as a stand-in for how font substitution is normally (automatically) handled by Windows (main UI font: Segoe UI, substitution for zh-CN: Microsoft YaHei)
- Chrome 57 looking pretty much the same, though I'm not sure if it actually uses system renderer or just emulates the default look
- Vivaldi using a serif font instead of the system default sans-serif, which not only breaks with the style of the main font it is substituting, but is barely eligible (after all, there is a reason that practically all UI fonts are sans-serif)
Here is an even better example, replacing Outlook with Notepad, and showing the same text in three windows:
Note that even the Western characters ("Unihan") are significantly less crisp and more blurred in Vivaldi than in Notepad (system default) and Chrome. This may be independent of the problem of wrong substitute font.