ID selectors

#id_value {
  /* … */
}

[id="id_value"] {
  /* … */
}

<p id="blue">This paragraph has a blue background.</p>
<p>This is just a regular paragraph.</p>

<!-- The next two paragraphs have id attributes
that contain characters which must be escaped in CSS -->
<p id="item?one">This paragraph has a pink background.</p>
<p id="123item">This paragraph has a yellow background.</p>

#blue {
  background-color: skyblue;
}

/* In the next two rules, the id attributes must be escaped */
#item\?one {
  background-color: pink;
}
#\00003123item {
  background-color: yellow;
}

#item?one {
  background-color: green;
}
#123item {
  background-color: green;
}