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HTML 

              
                <body>
        <!--ドロップダウンメニュー-->
        <div id="dropdownmenu">
            <div class="menu bigmenu">
                <!--ドロップダウン状態を判断するチェックボックス（非表示）-->
                <input type="checkbox" id="menu--toggle1" class="menu--toggle">
                
                <!--ドロップダウン状態を切り替えるラベル checkboxのIDを指定-->
                <label for="menu--toggle1" class="menu--label">
                    <!--メニューのタイトル-->
                    <h2 class="menu--label--title">商品情報</h2>
                    <!--アイコン ドロップダウン状態に応じて変化-->
                    <span class="menu--label--icon"></span>
                </label>
                
                <!--ドロップダウン項目-->
                <div class="menu--dropdownarea">
                    <div class="menu smallmenu">
                        <!--ドロップダウン状態を判断するチェックボックス（非表示）-->
                        <input type="checkbox" id="menu--toggle1-1" class="menu--toggle">
                        
                        <!--ドロップダウン状態を切り替えるラベル checkboxのIDを指定-->
                        <label for="menu--toggle1-1" class="menu--label">
                            <!--メニューのタイトル-->
                            <h3 class="menu--label--title">セール商品</h3>
                            <!--アイコン ドロップダウン状態に応じて変化-->
                            <span class="menu--label--icon"></span>
                        </label>
                        
                        <!--ドロップダウン項目-->
                        <div class="menu--dropdownarea">
                            <p>生活家電</p>
                            <p>情報家電</p>
                            <p>美容家電</p>
                        </div>
                       
                    </div>
                </div>
               
            </div>
        </div>
    </body>
!

CSS 

              
                *{margin:0;padding:0;box-sizing: border-box;transition:1s;}
#dropdownmenu{
    display:flex;
    .menu{


        > .menu--toggle{
            //display: none;
        }
        > .menu--label{
            display:flex;
            align-items: center;
            justify-content: space-around;
            background-color: lightblue;
            border: 1px solid lightgray;
            position:relative;

            > .menu--label--icon{
                height: 1rem;
                width:1rem;
                margin: 0.5rem;
                position: relative;
                
                &::before,&::after{
                    width:58%;
                }
                
                &::before, &::after{
                    content:"";
                    position: absolute;
                    top:0;
                    height: 5%;
                    background-color: black;
                    transition:1s
                }
                &::before{
                    left:0;
                    transform:rotate(45deg);
                }
                &::after{
                    right:0;
                    transform:rotate(-45deg);
                }
            }
        }
        > .menu--dropdownarea{
            background-color: lightgray;
        }

        > .menu--toggle:not(:checked){
            ~ .menu--label{
                > .menu--label--icon{
                    transform: rotate(-90deg);
                }
            }
            ~ .menu--dropdownarea{
                padding:0;
                border: none;
                *{
                    display: none;
                }
            }
        }
        > .menu--toggle:checked{
            ~ .menu--label{
                > .menu--label--icon{
                    transform: translateY(50%);
                }
            }
            ~ .menu--dropdownarea{
                border: 1px solid gray;
            }
        }
    }
    .bigmenu{
        width:25%;
        > .menu--label{
            background-color:lightblue;
        }
    }
    .smallmenu{
        width: 100%;
        position: relative;
        > .menu--label{
            background-color:aliceblue;
        }
        > .menu--dropdownarea{
            position: absolute;
            top:0;
            width: 100%;
            left:100%;
        }
    }
}
!

JS
!
999px

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