Font Weight

font-weight controls how thick text is: 100 is the thinnest, 400 is regular (normal), 500 is medium, 600 is semibold, 700 is bold, and 900 is the heaviest. In practice you only use three or four: 400 for body text, 500 or 600 for emphasis and subheadings, and 600 or 700 for large headings.

Weight is an effective way to build visual hierarchy, and it's subtler than a bigger font size. Make table headers, prices, and card titles a bit bolder, and users can find the key points at a glance. But bold everywhere means nothing stands out.

Note that the font itself has to include a weight for the browser to show it. For example, Arial on Windows only comes in regular and bold, so 500 and 600 may look the same as 400 or 700. When using web fonts, only load the weights you actually use, to keep the download small.