Some browsers have minimum font size settings in specific conditions
For security purposes, some browsers have a minimum font setting larger than zero by default in a specific condition.
When your website has text with a font size smaller than 10px or 12px, you should expect that that text might be rendered as 10px (even you set it as smaller, for example, 4px), causing unexpected trouble.
This page does deep research on the default setting in chrome (pdf snapshot).
Eventually, the default minimum font config is determined in
IDS_MINIMUM_FONT_SIZE variable. And it depends on the language the browser is installed with.
When I check with the Edge browser installed in Japanese, the default setting is 10px.
You can change the minimum font setting by going to the browser configuration page, searching for
フォント in Japanese) to find the appropriate config.
In my (English) chrome, however, the default config is