Two rectangles demonstrate a reusable D3 linearGradient definition, created in an SVG `defs` element. The first rectangle fills its interior with a diagonal red-to-blue gradient, while the second applies the same gradient to a four-pixel stroke, leaving its fill empty. Using d3-selection and d3 v5, the code sets `x1`, `x2`, `y1`, and `y2` attributes on the gradient and appends offset `stop` elements to control color transitions. The example shows how one gradient can style both fill and stroke in the same SVG scene.