How the first line of text can be indented?

The text-indent CSS property sets the length of empty space (indentation) that is put before lines of text in a block.

/* <length> values */
text-indent: 3mm;
text-indent: 40px;

/* <percentage> value
   relative to the containing block width */
text-indent: 15%;

/* Keyword values */
text-indent: 5em each-line;
text-indent: 5em hanging;
text-indent: 5em hanging each-line;

/* Global values */
text-indent: inherit;
text-indent: initial;
text-indent: unset;

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/text-indent