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HTML

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CSS

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <div id="cursor"></div>
<h1>
マウスカーソルをドットにする
</h1>

<div class="btn"><a href="">リンク</a></div>
!

CSS 

              
                html,body,a{
  cursor: none;
}
#cursor{
  transform: translate(0,0);
    pointer-events: none;
    position: fixed;
    top: -4px;    //座標調節（カーソル位置と円の中心を合わせる）
    left: -4px;   //座標調節（カーソル位置と円の中心を合わせる）
    width: 8px;   //カーソルの直径
    height: 8px;  //カーソルの直径
    background: rgba(0,0,0,0.75);
    border-radius: 50%;
    z-index: 999;
    transition: width .3s, height .3s, top .3s, left .3s;
  
  &.hov_{
    top: -12px;    //大きくなった分の座標調節
    left: -12px;   //大きくなった分の座標調節
    width: 32px;   //カーソルの直径
    height: 32px;  //カーソルの直径
    background: rgba(200,50,50,0.75);
  }
}

// 以下、カーソルに関係ない部分
.btn {
    display: block;
    width: 160px;
    margin: 40px auto;
    text-align: center;
    font-size: 14px;
    line-height: 1;

    a {
      display: block;
      color: #fff;
      text-decoration: none;
      padding: 8px;
      background-color: #007adf;
    }

}
!

JS 

              
                //準備
let cursorR = 4;  //カーソルの半径
const cursor = document.getElementById('cursor');  //カーソル用のdivを取得

//上記のdivタグをマウスに追従させる処理
document.addEventListener('mousemove', function (e) {
    cursor.style.transform = 'translate(' + e.clientX + 'px, ' + e.clientY + 'px)';
});

//リンクにホバー時はクラスをつける
const linkElem = document.querySelectorAll('a');
for (let i = 0; i < linkElem.length; i++) {
    linkElem[i].addEventListener('mouseover', function (e) {
        cursor.classList.add('hov_');
    });
    linkElem[i].addEventListener('mouseout', function (e) {
        cursor.classList.remove('hov_');      
    });
}
!
999px

Console