Question

Why all elements turn red? I only intend to turn <p> red.

<ul>
  <li>list item 1</li>
  <li>list item 2</li>
  <li>list item 3</li>
</ul>
<p>a paragraph</p>

<script>
$('li').add('p').css('background-color', 'red');
</script>

Answer 1

$('li') is selection all the li
$('li').add('p') is adding p tag into selected all the li
$('li').add('p').css('background-color', 'red') is changing all the li and p backgroundColor to red.

from jQuery.add API

Given a jQuery object that represents
a set of DOM elements, the .add()
method constructs a new jQuery object
from the union of those elements and
the ones passed into the method. The
argument to .add() can be pretty much
anything that $() accepts, including a
jQuery selector expression, references
to DOM elements, or an HTML snippet.