the new-ish <dialog> tag with a <form method="dialog"><button>Close</button></form> inside, combined with styling the pseudo-element ::backdrop, comes very close to being javascript-less built-in solution for modals. you still need js to trigger opening the dialog though, but that can be as simple as document.querySelector('#dialog').showModal()

apparently <dialog>'s still needs some accessibility/interaction improvements to be a first-class solution, but it's getting there: https://nolanlawson.com/2022/06/14/dialogs-and-shadow-dom-ca...