アコーディオン式モバイルサブメニュー

<div class="d03-01-11-stage">
          <nav class="d03-01-11-menu">
            <div class="d03-01-11-item">
              <button class="d03-01-11-top" type="button" aria-expanded="false">
                <span>サービス</span><span class="d03-01-11-ic"></span>
              </button>
              <div class="d03-01-11-sub">
                <div class="d03-01-11-sub-inner">
                  <a href="#" onclick="return false">Web制作</a>
                  <a href="#" onclick="return false">SEO対策</a>
                  <a href="#" onclick="return false">保守運用</a>
                </div>
              </div>
            </div>
            <a class="d03-01-11-flat" href="#" onclick="return false">実績</a>
            <a class="d03-01-11-flat" href="#" onclick="return false">会社概要</a>
          </nav>
        </div>

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          padding:13px 4px;background:none;border:none;cursor:pointer;
          font-size:13.5px;font-weight:700;color:#1f2937;font-family:inherit;
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        .d03-01-11-ic{position:relative;width:12px;height:12px;flex:none}
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        }
        .d03-01-11-ic::before{width:12px;height:2px}
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        .d03-01-11-item.open .d03-01-11-ic::after{transform:translate(-50%,-50%) scaleY(0)}
        .d03-01-11-sub{display:grid;grid-template-rows:0fr;transition:grid-template-rows .35s ease}
        .d03-01-11-item.open .d03-01-11-sub{grid-template-rows:1fr}
        .d03-01-11-sub-inner{min-height:0;overflow:hidden}
        .d03-01-11-sub-inner a{display:block;padding:9px 4px 9px 14px;font-size:12.5px;color:#475569;text-decoration:none}
        .d03-01-11-sub-inner a:hover{color:#2563eb}
        .d03-01-11-flat{display:block;padding:13px 4px;font-size:13.5px;font-weight:700;color:#1f2937;text-decoration:none;border-bottom:1px solid #e5e7eb}
        .d03-01-11-flat:hover{color:#2563eb}

(function(){
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        if(!item)return;
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          btn.setAttribute('aria-expanded',open?'true':'false');
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