If you want to ident the first line use the text-indent property, because only a limited number of CSS properties are relevant with ::first-line.

All font and background-related properties work, as well as color, word-spacing, letter-spacing, text-decoration, text-transform, line-height, text-shadow, text-decoration, text-decoration-color, text-decoration-line, text-decoration-style, and vertical-align.

Check the devTools