The tab-size property sets the width of the tab character (unicode U+0009). Default tab size value is 8;

Syntax:

tab-size: length | integer | inherit

integer – the number of spaces. The value must be positive.
length – the number with length unit (px, em, %)
Tab character normally is converted to spaces. When using tab-size property for other html tags than <pre> use white-space property and set to pre-wrap.

See the Pen
tab-size by majadc (@majadc)
on CodePen.

Resources:

MDN