Using font-family:

font-family: -apple-system; font-size:18;
font-family: system; font-size:18;

Using font:

font: -apple-system-body;
font: -apple-system-headline;
font: -apple-system-subheadline;
font: -apple-system-caption1;
font: -apple-system-caption2;
font: -apple-system-footnote;
font: -apple-system-short-body;
font: -apple-system-short-headline;
font: -apple-system-short-subheadline;
font: -apple-system-short-caption1;
font: -apple-system-short-footnote;
font: -apple-system-tall-body;