Should I put input elements inside a label element?

Behavior difference: clicking in the space between label and input

If you click on the space between the label and the input it activates the input only if the label contains the input.

This makes sense since in this case the space is just another character of the label.

div {
  border: 1px solid black;
}
label {
  border: 1px solid black;
  padding: 5px;
}
input {
  margin-right: 30px;
}

<p>Inside:</p>
<label>
  <input type="checkbox" />
  Label. Click between me and the checkbox.
</label>

<p>Outside:</p>
<input type="checkbox" id="check" />
<label for="check">Label. Click between me and the checkbox.</label>

Being able to click between label and box means that it is:

• easier to click
• less clear where things start and end

Bootstrap checkbox v3.3 examples use the input inside: http://getbootstrap.com/css/#forms Might be wise to follow them. But they changed their minds in v4.0 https://getbootstrap.com/docs/4.0/components/forms/#checkboxes-and-radios so I don't know what is wise anymore:

Checkboxes and radios use are built to support HTML-based form validation and provide concise, accessible labels. As such, our <input>s and <label>s are sibling elements as opposed to an <input> within a <label>. This is slightly more verbose as you must specify id and for attributes to relate the <input> and <label>.

UX question that discusses this point in detail: https://ux.stackexchange.com/questions/23552/should-the-space-between-the-checkbox-and-label-be-clickable

From the W3's HTML4 specification:

The label itself may be positioned before, after or around the associated control.

<label for="lastname">Last Name</label>
<input type="text" id="lastname" />

or

<input type="text" id="lastname" />
<label for="lastname">Last Name</label>

or

<label>
   <input type="text" name="lastname" />
   Last Name
</label>

Note that the third technique cannot be used when a table is being used for layout, with the label in one cell and its associated form field in another cell.

Either one is valid. I like to use either the first or second example, as it gives you more style control.

If you include the input tag in the label tag, you don't need to use the 'for' attribute.

That said, I don't like to include the input tag in my labels because I think they're separate, not containing, entities.

I prefer

<label>
  Firstname
  <input name="firstname" />
</label>

<label>
  Lastname
  <input name="lastname" />
</label>

over

<label for="firstname">Firstname</label>
<input name="firstname" id="firstname" />

<label for="lastname">Lastname</label>
<input name="lastname" id="lastname" />

Mainly because it makes the HTML more readable. And I actually think my first example is easier to style with CSS, as CSS works very well with nested elements.

But it's a matter of taste I suppose.

If you need more styling options, add a span tag.

<label>
  <span>Firstname</span>
  <input name="firstname" />
</label>

<label>
  <span>Lastname</span>
  <input name="lastname" />
</label>

Code still looks better in my opinion.