How to Set
1. in the body tag, set the font size to 62.5%. If no CSS is set in the body, the default size of the browser is 16px with 62.5%, the font size is 16px * 62.5% = 10px. You can use em to set the size easily. For example, if you need 12px, you only need to set 1.2em.
Fong-size: 1.4em;/* the size of H1 is equal to 14px */
2. If the font size in the body is set to 75%, the font size is 16px * 75% = 12px, and 1em is equivalent to 12px.
Fong-size: 1.5em;/* the size of H1 is equivalent to 18px */
The first method is better.
For an English website, you can also set it to small, X-small ......
Of course, the page design requirements will be higher.