Do not define CSSPropertyRule.initialValue as nullable

It should return empty string instead of null, when the descriptor is
omitted.

w3c/css-houdini-drafts#1115

Author: cdoublev

__tests__/stylesheet.js

@@ -1003,7 +1003,7 @@ describe('CSS grammar', () => {
         expect(rule.cssRules).toBeUndefined()
         expect(rule.style).toBeUndefined()
         expect(rule.color).toBeUndefined()
-        expect(rule.initialValue).toBeNull()
+        expect(rule.initialValue).toBe('')
         expect(rule.inherits).toBe('true')
         expect(rule.syntax).toBe('"*"')
     })
@@ -2118,7 +2118,7 @@ describe('CSSPropertyRule', () => {
         expect(rule.name).toBe('--custom')
         expect(rule.syntax).toBe('"*"')
         expect(rule.inherits).toBe('true')
-        expect(rule.initialValue).toBeNull()
+        expect(rule.initialValue).toBe('')
     })
 })
 describe('CSSScopeRule', () => {

lib/cssom/CSSPropertyRule-impl.js

@@ -17,9 +17,13 @@ class CSSPropertyRuleImpl extends CSSRuleImpl {
         super(globalObject, args, privateData)
         const { prelude, value } = privateData
         this.name = serializeCSSComponentValue(prelude)
-        value.declarations.forEach(({ name, value }) =>
-            this[cssPropertyToIDLAttribute(name)] = serializeCSSValue({ name, value }))
-        this.initialValue ??= null
+        value.declarations.forEach(({ name, value }) => {
+            let attribute = cssPropertyToIDLAttribute(name)
+            if (attribute === 'initialValue') {
+                attribute = `_${attribute}`
+            }
+            this[attribute] = serializeCSSValue({ name, value })
+        })
     }
 
     /**
@@ -28,14 +32,22 @@ class CSSPropertyRuleImpl extends CSSRuleImpl {
      * @see {@link https://drafts.css-houdini.org/css-properties-values-api-1/#serialize-a-csspropertyrule}
      */
     get cssText() {
-        const { inherits, initialValue, name, syntax } = this
+        const { inherits, _initialValue, name, syntax } = this
         let string = `@property ${name} { syntax: ${syntax}; inherits: ${inherits}; `
-        if (initialValue !== null) {
-            string += `initial-value: ${initialValue}; `
+        if (_initialValue !== undefined) {
+            string += `initial-value: ${_initialValue}; `
         }
         string += '}'
         return string
     }
+
+    /**
+     * @see {@link https://drafts.css-houdini.org/css-properties-values-api-1/#dom-csspropertyrule-initialvalue}
+     * @see {@link https://github.com/w3c/css-houdini-drafts/issues/1115}
+     */
+    get initialValue() {
+        return this._initialValue ?? ''
+    }
 }
 
 module.exports = {

lib/cssom/CSSPropertyRule.webidl

@@ -1,9 +1,10 @@
 
 // https://drafts.css-houdini.org/css-properties-values-api-1/#csspropertyrule
+// https://github.com/w3c/css-houdini-drafts/issues/1115
 [Exposed=Window]
 interface CSSPropertyRule : CSSRule {
     readonly attribute boolean inherits;
-    readonly attribute CSSOMString? initialValue;
+    readonly attribute CSSOMString initialValue;
     readonly attribute CSSOMString name;
     readonly attribute CSSOMString syntax;
 };