what is the different between bold and bolder??

Answer 1

Bold weight : Equivalent of 700 
Bolder: Bolder than the inherited font weight.

bolder is a relative font weight:

The 'bolder' and 'lighter' values select font weights that are relative to the weight inherited from the parent

bolder and lighterare even part of the official spec. How they are interpreted and displayed is up to the browser.

The fact that they appear the same visually is because most browsers don't properly support font weight variations beyond bold and normal.

Via ~ https://stackoverflow.com/questions/5592868/css-what-is-the-difference-between-bolder-and-bold-font-weight-styles

Answer 2

🔫 Rick Grimes
that's not the browsers who "don't properly support font weight variations", but the font family files wich do rarely includes a high number of weight variations (even not ever once).