:read-write

/* Selects any element that is editable */
/* Supported in Firefox with a prefix */
input:-moz-read-write {
  background-color: #ccc;
}

/* Supported in Blink/WebKit/Edge without prefix */
input:read-write {
  background-color: #ccc;
}

:read-write

<input type="text" value="Type whatever you want here.">
<input type="text" value="This is a read-only field." readonly>
<p contenteditable="true">You can edit this paragraph!</p>

input { min-width: 25em; }
input:-moz-read-write { background: cyan; }
input:read-write { background: cyan; }
p:-moz-read-write { background: lightgray; }
p:read-write { background: lightgray; }
p[contenteditable="true"] { color: blue; }