CSS background-size/position 示例

<!DOCTYPE html>
<html>
  <head>
    <style>
      .size-demo-grid {
        display: grid;
        grid-template-columns: repeat(3, 1fr);
        gap: 20px;
        margin-bottom: 18px;
        width: 100%;
      }
      .position-demo-grid {
        display: grid;
        grid-template-columns: repeat(3, 1fr);
        gap: 20px;
        margin-bottom: 18px;
        width: 100%;
      }
      @media (max-width: 900px) {
        .size-demo-grid,
        .position-demo-grid {
          grid-template-columns: repeat(2, 1fr);
        }
      }
      @media (max-width: 600px) {