CSS :optional

A decade ago HTML and CSS added the ability to, at least signal, validation of form fields. The required attribute helped inform users which fields were required, while pattern allowed developers to provide a regular expression to match against an <input>'s value. Targeting required fields and validation values with just CSS and HTML was very useful.

Did you know that CSS provides :optional to allow you to style form elements that aren't required?

input:optional, select:optional, textarea:optional {
  border: 1px solid #eee;
}

[required] {
  border: 1px solid red;
}

In a sense, it feels like :optional represents :not([required]), but :optional is limited to just form fields.