Preserve all empty variables

src/Parser.js

@@ -81,7 +81,7 @@ export function parse (value, root, parent, rule, rules, rulesets, pseudo, point
 					case 0: case 125: scanning = 0
 					// ;
 					case 59 + offset: if (ampersand == -1) characters = replace(characters, /\f/g, '')
-						if (property > 0 && (strlen(characters) - length || (variable === 0 && previous === 47)))
+						if (property > 0 && (strlen(characters) - length || variable === 0))
 							append(property > 32 ? declaration(characters + ';', rule, parent, length - 1, declarations) : declaration(replace(characters, ' ', '') + ';', rule, parent, length - 2, declarations), declarations)
 						break
 					// @ ;

test/Parser.js

@@ -170,6 +170,33 @@ describe('Parser', () => {
         --tw-brightness:/*!*/;
      `)
     ).to.equal(`.user{--tw-brightness:;}`)
+
+    expect(
+      stylis(`
+        --x:;
+        --y: ;
+      `)
+    ).to.equal(`.user{--x:;--y:;}`)
+
+    expect(
+      serialize(compile(`@keyframes foo{from{--x:red;}to{--x:;}}`), stringify)
+    ).to.equal(`@keyframes foo{from{--x:red;}to{--x:;}}`)
+
+    expect(
+      serialize(compile(`@keyframes foo{from{--x:red;}to{--x: ;}}`), stringify)
+    ).to.equal(`@keyframes foo{from{--x:red;}to{--x:;}}`)
+
+    expect(
+      stylis(`
+        .box { transform: translateX(20px) var(--extra-transform); }
+        .on { --extra-transform: scale(1.2); }
+        .off { --extra-transform: ; }
+      `)
+    ).to.equal([
+      `.user .box{transform:translateX(20px) var(--extra-transform);}`,
+      `.user .on{--extra-transform:scale(1.2);}`,
+      `.user .off{--extra-transform:;}`
+    ].join(''))
   })
 
   test('& in first selector within a comma-separated list', () => {