Why does an attribute CSS selector work both with and without apostrophes?

I know CSS Selectors should be generally of the following format:

tagName[attributeName='attributeValue']

For example Ask Question button on stackoverflow.com main page is sitting inside a div element matching the following CSS selector div[class='d-flex']
However I see with my Chrome DevTools that div[class=d-flex] locator is matching this element as well and it is found to be a correct locator!
So, my question is why the CSS selector expressions without apostrophes around the attribute values are still found as correct by the DevTools?

>Solution :

From the specification:

Attribute values must be <ident-token>s or <string-token>s

You never need to use apostrophes. There are useful only for values with spaces.

[data=ok] {
  height:100px;
  background:red;
}
[data="ok ok"] {
  height:100px;
  background:blue;
}

<div data=ok>

</div>

<div data="ok ok">

</div>