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HTML 

              
                <h1>What element can be selected with mouse</h1>
<div>
  <p>I met a problem when I tried to select a link in Chrome. I just can select the link. I noticed that, that link is a little special. When I click at the wrapper of that link, the link will be focused. So I think there may be some relation with the focus. So I wrote this article. This verify my thought. I also verify some other conditions.</p>
  <h2>Conclusion</h2>
  <p>An element can be selected only if its first parent/grandparent which can be focused (which tabindex=0) is focused.</p>
  <p><b>Note: </b>this may be not absolute accurate. Please leave a comment if you have any correction.</p>
</div>

<h2>Tests</h2>
<p>When you try to select an element (in this article, the element is a hyperlink), you should move your mouse after the element, then press your mouse and drag it to through the element. At last release the mouse. If <b>succeed to select</b>, the element will have a high contrast background and color. Like this <span class="selected">I am selected</span></p>
<p>In this article, the normal hyperlink looks like this <a href="#">normal</a>. The focused hyperlink looks like this <a href="#" class="focused">focused</a>. The normal wrapper looks likes this <span class="wrapper note-wrapper">normal</span>. The wrapper which is focused looks like this <span class="wrapper focused note-wrapper">focused</span></p>
<div tabindex="0" class="wrapper" onFocus="focusElement('link')">
  <h3>Test 1. When wrapper get the focus, it will focus the link inside</h3>
  <p><span>For test</span><a id="link" href="#">This <span class="not">can not</span> be selected in Chrome as the link is already focused, but <span class="can">can</span> be selected in Firefox</a></p>
</div>
<div tabindex="0" class="wrapper" onFocus="focusElement('link')">
  <h3>Test 2. When wrapper get the focus, it will focus the link out of this wrapper (more specifically, it will focus the link in Test 1)</h3>
  <p><span>For test</span><a href="#">This <span class="not">can not</span> be selected in Chrome as the focus is out of its parent, but <span class="can">can</span> be selected in Firefox (not perfect)</a></p>
</div>
<div tabindex="0" class="wrapper" onFocus="focusElement('siblingLink')">
  <h3>Test 3. When wrapper get the focus, it will focus a link inside</h3>
  <p><a href="#" id="siblingLink">The sibling Link that will be focused automatically</a></p>
  <p><span>For test</span><a href="#">This <span class="not">can not</span> be selected in Chrome as its sibling is focused, but <span class="can">can</span> be selected in Firefox (not perfect)</a></p>
</div>
<div tabindex="0" class="wrapper" id="parentWrapper">
  <h3>Test 4. When wrapper get the focus, it will focus its parent</h3>
  <div tabindex="0" class="wrapper" onFocus="focusElement('parentWrapper')">
    <p><span>For test</span><a href="#">This <span class="not">can not</span> be selected in Chrome as its grandparent wrapper is focused, but <span class="can">can</span> be selected in Firefox (not perfect)</a></p>
  </div>
</div>
<div tabindex="0" class="wrapper" id="parentWrapper">
  <h3>Test 5. When wrapper get the focus, it will focus its parent</h3>
  <p>This is a little different with Test 4. The parent of following hyperlink have no property tabindex.</p>
  <div class="wrapper" onFocus="focusElement('parentWrapper')">
    <p><span>For test</span><a href="#">This <span class="can">can</span> be selected in Chrome even though its grandparent wrapper is focused, because its parent cannot be focused (without tabindex=0)</a></p>
  </div>
</div>
<div tabindex="0" class="wrapper" id="selfWrapper" onFocus="focusElement('selfWrapper')">
  <h3>Test 6. When wrapper get the focus, it will focus itself (manually)</h3>
  <p><span>For test</span><a href="#">This <span class="can">can</span> be selected as its parent wrapper is focused</a></p>
</div>
<div tabindex="0" class="wrapper">
  <h3>Test 7. When wrapper get the focus, do nothing</h3>
  <p><span>For test</span><a href="#">This <span class="can">can</span> be selected as its parent wrapper is focused</a></p>
</div>
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CSS 

              
                .wrapper {
  padding: 10px;
  border: 1px solid #cdcdcd;
  margin-bottom: 10px;
}
.wrapper:focus,
.wrapper.focused {
  border: 1px solid #f00;
  outline: none;
}
.wrapper a {
  text-decoration: none;
  outline: none;
}
.wrapper a:focus,
.wrapper a:hover,
.wrapper a:active,
a.focused {
  color: #f00;
  text-decoration: underline;
}
.wrapper span {
  display: inline-block;
  padding-left: 10px;
  padding-right: 10px;
  margin-right: 10px;
  background-color: #00f;
  color: #fff;
}
span.can {
  background-color: transparent;
  color: #096;
  padding: 0;
  margin: 0;
}
span.not {
  background-color: transparent;
  color: #f00;
  padding: 0;
  margin: 0;
}
span.selected {
  background-color: #09f;
  color: #fff;
}
.note-wrapper {
  display: inline-block;
}
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JS 

              
                var focusElement = function (id) {
  var element = document.getElementById(id);
  element && element.focus();
};
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999px

Console