What does a CSS selector in square brackets select in HTML?

It's not a class, you encountered a so called attribute selector. It matches every html element that has got that attribute set, whatever the value. I.e. <section text-uppercase="true">, <div text-uppercase="something">, <nav text-uppercase>…

Look at the reference provided on the link above for more advanced usage scenarios.

[text-uppercase] {
  text-transform: uppercase;
}

<span text-uppercase>hello</span>

For the selector to work:
<div text-uppercase></div>

[text-uppercase] selector matches an attribute on a tag.