Generally, CSS selectors refer to markup or, in some cases, to element properties as set with scripting (client-side JavaScript), rather than user actions. For example, :empty matches element with empty content in markup; all input elements are unavoidably empty in this sense. The selector [value=""] tests whether the element has the value attribute in markup and has the empty string as its value. And :checked and :indeterminate are similar things. They are not affected by actual user input.

The selector [value=""] tests whether the element has the value attribute in markup and has the empty string as its value.

This sibling combinator is similar to X + Y, however, it's less strict. While an adjacent selector (ul + p) will only select the first element that is immediately preceded by the former selector, this one is more generalized. It will select, referring to our example above, any p elements, as long as they follow a ul.

In the case above, :checked is a reasonable thing to search for, but [checked] is not.

CSS syntax is awesome for two reasons: It is an order of magnitude faster and less resource intensive than the more complex XPath. When what you want to find can be found with a css selector, a corresponding XPath query doing the same would most of the time be much longer and harder to read.