I was building wireframes for a website with HTML & CSS. Since it's a prototype, not all actions are functional. When a visitor reviewing the prototype tries to click something that isn't hooked up, I wanted to clarify what they could interact with. This also allows visitors to click anywhere on the page to highlight what they can click.

In the past, I've used JavaScript to add an outline to clickable elements when something non-interactive was clicked. But with the :has and :is selectors, this is doable with plain CSS.

html:active:not(:has(a:active, button:active, label:active)) :is(a, button, label) {
  outline: 2px solid blue;
}

How it works:

html:active will match whenever you hold down your mouse on the page
:not(:has(a:active, button:active, label:active)) will not match when you're holding down your mouse on an a, button, or label element, to avoid the outline from appearing when the user clicks something that is functional
:is(a, button, label) matches all a, button, and label elements on the page

View a demo on CodePen.