It’s CSS shorthand.

font-size/line-height font-family

An Em is a typographical unit that basically means “the current font size”, so it actually nests in CSS/HTML.

Default browser font size tends to be a 16pt pixel equivalent. So in this case this is 80% of that.

Line height is obvious I think, but yeah - it’s in a % of the current font size.

They’re just standard typography syntax (the “size/height” bit anyhow) and units moved into CSS with no change.