stroke-linecap用來描述線段<line>的端點呈現方式，提供三種屬性
stroke-linecap: butt預設值，端點不外擴也不變形stroke-linecap: round端點外擴並呈現圓角stroke-linecap: square端點外擴並呈現方角
（中間畫一條<path>比較好觀察外擴情形）
butt（預設值）
svg(viewbox="-50 -50 100 100" width="500")
line(x1=-40 y1=0 x2=40 y2=0 stroke="black" stroke-width=8 stroke-linecap="butt")
path(d="M-40, 00 L40, 0" stroke="white")round
svg(viewbox="-50 -50 100 100" width="500")
line(x1=-40 y1=0 x2=40 y2=0 stroke="black" stroke-width=8 stroke-linecap="round")
path(d="M-40, 00 L40, 0" stroke="white")square
svg(viewbox="-50 -50 100 100" width="500")
line(x1=-40 y1=0 x2=40 y2=0 stroke="black" stroke-width=8 stroke-linecap="square")
path(d="M-40, 00 L40, 0" stroke="white")