Fix user-select: contain

Author: ai

lib/hacks/user-select.js

@@ -7,6 +7,11 @@ class UserSelect extends Declaration {
   insert(decl, prefix, prefixes) {
     if (decl.value === 'all' && prefix === '-ms-') {
       return undefined
+    } else if (
+      decl.value === 'contain' &&
+      (prefix === '-moz-' || prefix === '-webkit-')
+    ) {
+      return undefined
     } else {
       return super.insert(decl, prefix, prefixes)
     }

test/cases/user-select.out.css

@@ -5,9 +5,8 @@ a {
 }
 
 b {
-  -webkit-user-select: contain;
-      -ms-user-select: element;
-          user-select: contain;
+  -ms-user-select: element;
+      user-select: contain;
 }
 
 .all {