Custom CSS checkbox only working when label has "for" attribute

Question

I found some really weird things I can't explain. I tried making custom checkboxes using CSS by following this codepen.

I copied most of it, but for some reason it didn't work. After checking my code, I noticed that I didn't use the id attribute on my checkbox and neither did I have a for attribute on my label.

For example:

/* Base for label styling */
[type="checkbox"]:not(:checked),
[type="checkbox"]:checked {
  position: absolute;
  left: -9999px;
}
[type="checkbox"]:not(:checked) + label,
[type="checkbox"]:checked + label {
  position: relative;
  padding-left: 25px;
  cursor: pointer;
}

/* checkbox aspect */
[type="checkbox"]:not(:checked) + label:before,
[type="checkbox"]:checked + label:before {
  content: '';
  position: absolute;
  left:0; top: 2px;
  width: 17px; height: 17px;
  border: 1px solid #aaa;
  background: #f8f8f8;
  border-radius: 3px;
  box-shadow: inset 0 1px 3px rgba(0,0,0,.3)
}
/* checked mark aspect */
[type="checkbox"]:not(:checked) + label:after,
[type="checkbox"]:checked + label:after {
  content: '✔';
  position: absolute;
  top: 0; left: 4px;
  font-size: 14px;
  color: #09ad7e;
  transition: all .2s;
}
/* checked mark aspect changes */
[type="checkbox"]:not(:checked) + label:after {
  opacity: 0;
  transform: scale(0);
}
[type="checkbox"]:checked + label:after {
  opacity: 1;
  transform: scale(1);
}
/* disabled checkbox */
[type="checkbox"]:disabled:not(:checked) + label:before,
[type="checkbox"]:disabled:checked + label:before {
  box-shadow: none;
  border-color: #bbb;
  background-color: #ddd;
}
[type="checkbox"]:disabled:checked + label:after {
  color: #999;
}
[type="checkbox"]:disabled + label {
  color: #aaa;
}
/* accessibility */
[type="checkbox"]:checked:focus + label:before,
[type="checkbox"]:not(:checked):focus + label:before {
  border: 1px dotted blue;
}

/* hover style just for information */
label:hover:before {
  border: 1px solid #4778d9!important;
}

body {
  font-family: "Open sans", "Segoe UI", "Segoe WP", Helvetica, Arial, sans-serif;
  color: #777;
}

<form action="#">
  <p>
    <input type="checkbox" />
    <label>Red</label>
  </p>
  <p>
    <input type="checkbox" id="test2" checked="checked" />
    <label for="test2">Yellow</label>
  </p>
  <p>
    <input type="checkbox" id="test3" checked="checked" disabled="disabled" />
    <label for="test3">Green</label>
  </p>
    <p>
      <input type="checkbox" id="test4" disabled="disabled" />
      <label for="test4">Brown</label>
  </p>
</form>

For some reason, this custom styled checkboxes only work if the for and id attribute are specified. If I leave them away, I can no longer check/uncheck the checkbox. If you look at the snippet above, you will notice that you can't check/uncheck the "Red" checkbox, but you can check/uncheck the yellow one. I tried it on both Google Chrome and Mozilla Firefox (on Ubuntu), same behavior in both browsers.

I thought those attribute where pure semantics, but it seems that it causes some other things to happen as well.

I'm happy I got it to work, but I'm still confused and I don't know why it doesn't work without the attributes.

Answer 1

The for attribute in the label tells the browser to move the focus to the input whose ID is identical to the value. This makes the label a labelled control for that input element. This question has also been addressed previously on SO: What does "for" attribute do in HTML <label> tag?

When you remove the for attribute on the <label> element or the id attribute on the <input> element, you are decoupling both elements and therefore preventing the click event on the <label> from propagating into a focus + click event on the corresponding checkbox.