Historically, printers relied on proportional metal slugs carved by hand to balance uppercase titling against dense paragraph text. Today, in the era of responsive high-DPI displays, that delicate choreography belongs to the CSS box model and Google Fonts CDN.
"A typeface is the clothing that words wear; make sure your headings and body wear complimentary outfits."
By establishing a mathematical ratio—pairing high-contrast serifs with low-contrast neutral sans-serifs—designers can direct the reader's eye seamlessly down the page.