<!-- WILL match -->
<div data-modal="open">I love footy</div>

<!-- WILL match -->
<aside class='closed' data-modal='open'></aside>

<!-- Will NOT match -->
<div data-modal="false">Wrong value</div>

<!-- Will NOT match -->
<div data-modal>No value</div>

<!-- Will NOT match -->
<div data-modal-open>Wrong attribute</div>

[data-modal="open"] {
  
  color:blue;
  font-size:large

}