• 五边形：
  clip-path: polygon(50% 0%, 100% 38%, 82% 100%, 18% 100%, 0% 38%);
• 六边形：
  clip-path: polygon(50% 0%, 100% 25%, 100% 75%, 50% 100%, 0% 75%, 0% 25%);
• 七边形：
  clip-path: polygon(50% 0%, 90% 20%, 100% 60%, 75% 100%, 25% 100%, 0% 60%, 10% 20%);

反正用 clip-path