[disabled] => :disabled

:disabled is supported in IE9+ and all modern browsers

scss/_forms.scss

@@ -140,7 +140,7 @@ output {
   // HTML5 says that controls under a fieldset > legend:first-child won't be
   // disabled if the fieldset is disabled. Due to implementation difficulty, we
   // don't honor that edge case; we style them as disabled anyway.
-  &[disabled],
+  &:disabled,
   &[readonly],
   fieldset[disabled] & {
     cursor: $cursor-disabled;
@@ -263,7 +263,7 @@ input[type="search"] {
 // Note: Neither radios nor checkboxes can be readonly.
 input[type="radio"],
 input[type="checkbox"] {
-  &[disabled],
+  &:disabled,
   &.disabled,
   fieldset[disabled] & {
     cursor: $cursor-disabled;