Why does element[class=noquotes] work?

Question

As per MDN attribute selector in css has the following syntax:

div[attr="actualValue"]

Or like this example:

<div class="test"></div>  

div[class="test"]

But I noticed that the quotes around the test are not needed as:

div[class=test]

Example:

div[class=test] {
  background: #6ea7b2;
}

<div class="test">I am a div</div>

Why is this? Does w3c spec allow attribute selection without quotes around the value?

Answer 1

From the specification:

Attribute values must be CSS identifiers or strings.

"test" is a string, which is fine.

test is an identifier, which is fine.