Font family

The headings are using ui-sans-serif. The body text is using the ui-serif. The code and the pre elements are using ui-monospace.

Serif

In typography, a serif is a small line or stroke regularly attached to the end of a larger stroke in a letter or symbol within a particular font or family of fonts.

Reference:

Serif - Wikipedia

Sans-serif

In typography and lettering, a sans-serif, sans serif, gothic, or simply sans letterform is one that does not have extending features called "serifs" at the end of strokes. Sans-serif typefaces tend to have less stroke width variation than serif typefaces. They are often used to convey simplicity and modernity or minimalism.

Reference:

Sans-serif - Wikipedia

Monospace

monospaced font, also called a fixed-pitch, fixed-width, or non-proportional font, is a font whose letters and characters each occupy the same amount of horizontal space. This contrasts with variable-width fonts, where the letters and spacings have different widths.

        
console.log("Hello world");
document.body.textContent = "Hello World!";
        
      

Reference

Monospaced font - Wikipedia