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HTML

              
                <div class="example1">サンプル1</div>
<div class="example2"><mark>サンプル2</mark></div>

<div id="wrapper">
  <p class="example3" id="example3">サンプル<strong>3。</strong></p>
</div>

<p>Author : <a href="http://neo.s21.xrea.com/">Neo</a></p>
              
            
!

CSS

              
                html,
body {
  margin: 0;
  padding: 0; /* 基本は margin だけで十分だけど… */
  width: 100%;
  height: 100%;
}

/* 以下本質に関係ないサンプル用の要素のスタイリング */

.example1 {
  position: absolute;
  top: 50px;
  left: 50px;
  width: 150px;
  height: 150px;
  border: 1px solid #f00;
  background: #fff;
}

.example2 {
  position: absolute;
  top: 100px;
  left: 100px;
  width: 150px;
  height: 150px;
  border: 1px solid #00f;
  background: rgba(255, 255, 255, .6);
}

#wrapper {
  margin-top: 300px;
  margin-left: 1em;
}
              
            
!

JS

              
                document.addEventListener('DOMContentLoaded', (loadEvent) => {
  // ポインタ要素を生成しておく
  const pointer = document.createElement('div');
  // position: fixed を使うと完全にフローして画面外にはみ出てもスクロールバーが表示されたりしなくなる
  pointer.setAttribute('style', 'display: inline-block; position: fixed; top: -1000px; border: 1px solid #ccc; border-radius: 4px; padding: 10px; background: #fff; opacity: .7;');
  // スクロールバーは html 要素が出すので body 要素配下にいるポインタ要素はスクロールバーの下に隠れる
  document.body.appendChild(pointer);
  
  // 直前にポイントした要素を控えておく
  let previousElement;
  
  // 引数の要素の CSS セレクタ表現を作る
  const getQuerySelector = (elem) => {
    const tagName = elem.tagName.toLowerCase();
    let querySelector = tagName;
    if(elem.id.trim()) {
      querySelector += '#' + elem.id.trim();
    }
    if(elem.className.trim()) {
      querySelector += '.' + Array.apply(null, elem.classList).join('.');
    }
    return querySelector;
  };
  
  // ポインタの位置にある要素を取得し CSS セレクタ表現を表示する
  const getElementFromPoint = (event) => {
    // ポインタの位置を設定する
    pointer.style.top = (event.y + 10) + 'px';
    pointer.style.left = (event.x + 5) + 'px';
    
    // 指定の座標位置にある要素を取得する
    const elementFromPoint = document.elementFromPoint(event.x, event.y);
    // 要素がないか直前の要素と同じなら中止
    if(!elementFromPoint || elementFromPoint === previousElement) {
      return;
    }
    // 直前の要素として控えておく
    previousElement = elementFromPoint;
    
    // 当該要素の CSS セレクタ表現を作る
    let parentElement = elementFromPoint;
    let tagName = elementFromPoint.tagName.toLowerCase();
    let querySelector = getQuerySelector(elementFromPoint);
    // 親要素に遡って CSS セレクタ表現を作る
    while(tagName !== 'html') {
      parentElement = parentElement.parentElement;
      tagName = parentElement.tagName.toLowerCase();
      querySelector = getQuerySelector(parentElement) + ' > ' + querySelector;
    }
    
    // ポインタ要素に表示する
    pointer.textContent = querySelector;
  }
  
  document.addEventListener('mousemove', getElementFromPoint); // マウス操作中
  document.addEventListener('click'    , getElementFromPoint); // マウスボタン押下時
});
              
            
!
999px

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