diff --git a/src/browser/tests/css/stylesheet.html b/src/browser/tests/css/stylesheet.html
index abc1ed92..ec14f4fc 100644
--- a/src/browser/tests/css/stylesheet.html
+++ b/src/browser/tests/css/stylesheet.html
@@ -205,3 +205,54 @@
     testing.expectEqual('', style.getPropertyPriority('content'));
 }
 </script>
+
+<script id="CSSStyleDeclaration_style_syncs_to_attribute">
+{
+    // JS style modifications must be reflected in getAttribute.
+    const div = document.createElement('div');
+
+    // Named property assignment (element.style.X = ...)
+    div.style.opacity = '0';
+    testing.expectEqual('opacity: 0;', div.getAttribute('style'));
+
+    // Update existing property
+    div.style.opacity = '1';
+    testing.expectEqual('opacity: 1;', div.getAttribute('style'));
+
+    // Add a second property
+    div.style.color = 'red';
+    testing.expectTrue(div.getAttribute('style').includes('opacity: 1'));
+    testing.expectTrue(div.getAttribute('style').includes('color: red'));
+
+    // removeProperty syncs back
+    div.style.removeProperty('opacity');
+    testing.expectTrue(!div.getAttribute('style').includes('opacity'));
+    testing.expectTrue(div.getAttribute('style').includes('color: red'));
+
+    // setCssText syncs back
+    div.style.cssText = 'filter: blur(0px)';
+    testing.expectEqual('filter: blur(0px);', div.getAttribute('style'));
+
+    // setCssText with empty string clears attribute
+    div.style.cssText = '';
+    testing.expectEqual('', div.getAttribute('style'));
+}
+</script>
+
+<script id="CSSStyleDeclaration_outerHTML_reflects_style_changes">
+{
+    // outerHTML must reflect JS-modified styles (regression test for
+    // DOM serialization reading stale HTML-parsed attribute values).
+    const div = document.createElement('div');
+    div.setAttribute('style', 'filter:blur(10px);opacity:0');
+
+    div.style.filter = 'blur(0px)';
+    div.style.opacity = '1';
+
+    const html = div.outerHTML;
+    testing.expectTrue(html.includes('filter: blur(0px)'));
+    testing.expectTrue(html.includes('opacity: 1'));
+    testing.expectTrue(!html.includes('blur(10px)'));
+    testing.expectTrue(!html.includes('opacity:0'));
+}
+</script>
diff --git a/src/browser/webapi/css/CSSStyleDeclaration.zig b/src/browser/webapi/css/CSSStyleDeclaration.zig
index 8380b6f3..23b04170 100644
--- a/src/browser/webapi/css/CSSStyleDeclaration.zig
+++ b/src/browser/webapi/css/CSSStyleDeclaration.zig
@@ -33,10 +33,27 @@ _properties: std.DoublyLinkedList = .{},
 _is_computed: bool = false,
 
 pub fn init(element: ?*Element, is_computed: bool, page: *Page) !*CSSStyleDeclaration {
-    return page._factory.create(CSSStyleDeclaration{
+    const self = try page._factory.create(CSSStyleDeclaration{
         ._element = element,
         ._is_computed = is_computed,
     });
+
+    // Parse the element's existing style attribute into _properties so that
+    // subsequent JS reads and writes see all CSS properties, not just newly
+    // added ones.  Computed styles have no inline attribute to parse.
+    if (!is_computed) {
+        if (element) |el| {
+            if (el.getAttributeSafe(String.wrap("style"))) |attr_value| {
+                var it = CssParser.parseDeclarationsList(attr_value);
+                while (it.next()) |declaration| {
+                    const priority: ?[]const u8 = if (declaration.important) "important" else null;
+                    try self.setPropertyImpl(declaration.name, declaration.value, priority, page);
+                }
+            }
+        }
+    }
+
+    return self;
 }
 
 pub fn length(self: *const CSSStyleDeclaration) u32 {
@@ -76,8 +93,13 @@ pub fn getPropertyPriority(self: *const CSSStyleDeclaration, property_name: []co
 }
 
 pub fn setProperty(self: *CSSStyleDeclaration, property_name: []const u8, value: []const u8, priority_: ?[]const u8, page: *Page) !void {
+    try self.setPropertyImpl(property_name, value, priority_, page);
+    try self.syncStyleAttribute(page);
+}
+
+fn setPropertyImpl(self: *CSSStyleDeclaration, property_name: []const u8, value: []const u8, priority_: ?[]const u8, page: *Page) !void {
     if (value.len == 0) {
-        _ = try self.removeProperty(property_name, page);
+        _ = try self.removePropertyImpl(property_name, page);
         return;
     }
 
@@ -110,6 +132,12 @@ pub fn setProperty(self: *CSSStyleDeclaration, property_name: []const u8, value:
 }
 
 pub fn removeProperty(self: *CSSStyleDeclaration, property_name: []const u8, page: *Page) ![]const u8 {
+    const result = try self.removePropertyImpl(property_name, page);
+    try self.syncStyleAttribute(page);
+    return result;
+}
+
+fn removePropertyImpl(self: *CSSStyleDeclaration, property_name: []const u8, page: *Page) ![]const u8 {
     const normalized = normalizePropertyName(property_name, &page.buf);
     const prop = self.findProperty(normalized) orelse return "";
 
@@ -121,6 +149,14 @@ pub fn removeProperty(self: *CSSStyleDeclaration, property_name: []const u8, pag
     return old_value;
 }
 
+// Serialize current properties back to the element's style attribute so that
+// DOM serialization (outerHTML, getAttribute) reflects JS-modified styles.
+fn syncStyleAttribute(self: *CSSStyleDeclaration, page: *Page) !void {
+    const element = self._element orelse return;
+    const css_text = try self.getCssText(page);
+    try element.setAttributeSafe(String.wrap("style"), String.wrap(css_text), page);
+}
+
 pub fn getFloat(self: *const CSSStyleDeclaration, page: *Page) []const u8 {
     return self.getPropertyValue("float", page);
 }
@@ -154,8 +190,9 @@ pub fn setCssText(self: *CSSStyleDeclaration, text: []const u8, page: *Page) !vo
     var it = CssParser.parseDeclarationsList(text);
     while (it.next()) |declaration| {
         const priority: ?[]const u8 = if (declaration.important) "important" else null;
-        try self.setProperty(declaration.name, declaration.value, priority, page);
+        try self.setPropertyImpl(declaration.name, declaration.value, priority, page);
     }
+    try self.syncStyleAttribute(page);
 }
 
 pub fn format(self: *const CSSStyleDeclaration, writer: *std.Io.Writer) !void {