diff --git a/src/browser/tests/element/styles.html b/src/browser/tests/element/styles.html
index 89946038..e4b7c489 100644
--- a/src/browser/tests/element/styles.html
+++ b/src/browser/tests/element/styles.html
@@ -127,3 +127,74 @@
   testing.expectEqual('important', div.style.getPropertyPriority('color'));
 }
 </script>
+
+<script id="defaultVisibilityProperties">
+{
+  // Test default values for visibility-related properties
+  const div = document.createElement('div');
+  const span = document.createElement('span');
+  const anchor = document.createElement('a');
+
+  // Test display defaults on element.style
+  testing.expectEqual('block', div.style.getPropertyValue('display'));
+  testing.expectEqual('inline', span.style.getPropertyValue('display'));
+  testing.expectEqual('inline', anchor.style.getPropertyValue('display'));
+
+  // Test visibility default
+  testing.expectEqual('visible', div.style.getPropertyValue('visibility'));
+
+  // Test opacity default
+  testing.expectEqual('1', div.style.getPropertyValue('opacity'));
+
+  // Test that explicit values override defaults
+  div.style.setProperty('display', 'flex');
+  testing.expectEqual('flex', div.style.getPropertyValue('display'));
+  testing.expectEqual(1, div.style.length);
+
+  // Test that removing property returns to default
+  div.style.setProperty('display', '');
+  testing.expectEqual('block', div.style.getPropertyValue('display'));
+  testing.expectEqual(0, div.style.length);
+
+  // Test that other properties still return empty when not set
+  testing.expectEqual('', div.style.getPropertyValue('color'));
+  testing.expectEqual('', div.style.getPropertyValue('width'));
+}
+</script>
+
+<script id="computedStyleDefaults">
+{
+  // Test that getComputedStyle returns default visibility properties
+  const div = document.createElement('div');
+  const span = document.createElement('span');
+  const anchor = document.createElement('a');
+  document.body.appendChild(div);
+  document.body.appendChild(span);
+  document.body.appendChild(anchor);
+
+  // Test element.style defaults
+  testing.expectEqual('block', div.style.display);
+  testing.expectEqual('inline', span.style.display);
+  testing.expectEqual('inline', anchor.style.display);
+  testing.expectEqual('visible', div.style.visibility);
+  testing.expectEqual('1', div.style.opacity);
+
+  // Test getComputedStyle with getPropertyValue
+  const divStyle = window.getComputedStyle(div);
+  const spanStyle = window.getComputedStyle(span);
+  const anchorStyle = window.getComputedStyle(anchor);
+
+  testing.expectEqual('block', divStyle.getPropertyValue('display'));
+  testing.expectEqual('inline', spanStyle.getPropertyValue('display'));
+  testing.expectEqual('inline', anchorStyle.getPropertyValue('display'));
+  testing.expectEqual('visible', divStyle.getPropertyValue('visibility'));
+  testing.expectEqual('1', divStyle.getPropertyValue('opacity'));
+
+  // Test getComputedStyle with named property access (camelCase)
+  testing.expectEqual('block', divStyle.display);
+  testing.expectEqual('inline', spanStyle.display);
+  testing.expectEqual('inline', anchorStyle.display);
+  testing.expectEqual('visible', divStyle.visibility);
+  testing.expectEqual('1', divStyle.opacity);
+}
+</script>
diff --git a/src/browser/webapi/Window.zig b/src/browser/webapi/Window.zig
index b9060aaf..6f9a03c4 100644
--- a/src/browser/webapi/Window.zig
+++ b/src/browser/webapi/Window.zig
@@ -40,7 +40,7 @@ const MessageEvent = @import("event/MessageEvent.zig");
 const MediaQueryList = @import("css/MediaQueryList.zig");
 const storage = @import("storage/storage.zig");
 const Element = @import("Element.zig");
-const CSSStyleDeclaration = @import("css/CSSStyleDeclaration.zig");
+const CSSStyleProperties = @import("css/CSSStyleProperties.zig");
 const CustomElementRegistry = @import("CustomElementRegistry.zig");
 
 const Window = @This();
@@ -319,8 +319,8 @@ pub fn matchMedia(_: *const Window, query: []const u8, page: *Page) !*MediaQuery
     });
 }
 
-pub fn getComputedStyle(_: *const Window, _: *Element, page: *Page) !*CSSStyleDeclaration {
-    return CSSStyleDeclaration.init(null, page);
+pub fn getComputedStyle(_: *const Window, element: *Element, page: *Page) !*CSSStyleProperties {
+    return CSSStyleProperties.init(element, page);
 }
 
 pub fn postMessage(self: *Window, message: js.Object, target_origin: ?[]const u8, page: *Page) !void {
diff --git a/src/browser/webapi/css/CSSStyleDeclaration.zig b/src/browser/webapi/css/CSSStyleDeclaration.zig
index 536fa737..afa83202 100644
--- a/src/browser/webapi/css/CSSStyleDeclaration.zig
+++ b/src/browser/webapi/css/CSSStyleDeclaration.zig
@@ -55,7 +55,9 @@ pub fn item(self: *const CSSStyleDeclaration, index: u32) []const u8 {
 
 pub fn getPropertyValue(self: *const CSSStyleDeclaration, property_name: []const u8, page: *Page) []const u8 {
     const normalized = normalizePropertyName(property_name, &page.buf);
-    const prop = self.findProperty(normalized) orelse return "";
+    const prop = self.findProperty(normalized) orelse {
+        return getDefaultPropertyValue(self, normalized);
+    };
     return prop._value.str();
 }
 
@@ -192,6 +194,54 @@ fn normalizePropertyName(name: []const u8, buf: []u8) []const u8 {
     return std.ascii.lowerString(buf, name);
 }
 
+fn getDefaultPropertyValue(self: *const CSSStyleDeclaration, normalized_name: []const u8) []const u8 {
+    if (std.mem.eql(u8, normalized_name, "visibility")) {
+        return "visible";
+    }
+    if (std.mem.eql(u8, normalized_name, "opacity")) {
+        return "1";
+    }
+    if (std.mem.eql(u8, normalized_name, "display")) {
+        const element = self._element orelse return "";
+        return getDefaultDisplay(element);
+    }
+
+    return "";
+}
+
+fn getDefaultDisplay(element: *const Element) []const u8 {
+    switch (element._type) {
+        .html => |html| {
+            return switch (html._type) {
+                .anchor, .br => "inline",
+                .body, .div, .p, .heading, .form, .button, .canvas, .dialog, .embed, .head, .html, .hr, .iframe, .img, .input, .li, .link, .meta, .ol, .option, .script, .select, .slot, .style, .template, .textarea, .title, .ul, .media => "block",
+                .generic, .custom, .unknown, .data => blk: {
+                    const tag = element.getTagNameLower();
+                    if (isInlineTag(tag)) break :blk "inline";
+                    break :blk "block";
+                },
+            };
+        },
+        .svg => return "inline",
+    }
+}
+
+fn isInlineTag(tag_name: []const u8) bool {
+    const inline_tags = [_][]const u8{
+        "abbr",  "b",    "bdi",    "bdo",  "cite", "code", "dfn",
+        "em",    "i",    "kbd",    "mark", "q",    "s",    "samp",
+        "small", "span", "strong", "sub",  "sup",  "time", "u",
+        "var",   "wbr",
+    };
+
+    for (inline_tags) |inline_tag| {
+        if (std.mem.eql(u8, tag_name, inline_tag)) {
+            return true;
+        }
+    }
+    return false;
+}
+
 pub const Property = struct {
     _name: String,
     _value: String,
diff --git a/src/browser/webapi/css/CSSStyleProperties.zig b/src/browser/webapi/css/CSSStyleProperties.zig
index 6a92295a..bb1ec70c 100644
--- a/src/browser/webapi/css/CSSStyleProperties.zig
+++ b/src/browser/webapi/css/CSSStyleProperties.zig
@@ -106,6 +106,8 @@ fn isKnownCSSProperty(dash_case: []const u8) bool {
         .{ "width", {} },
         .{ "height", {} },
         .{ "display", {} },
+        .{ "visibility", {} },
+        .{ "opacity", {} },
         .{ "position", {} },
         .{ "top", {} },
         .{ "bottom", {} },
@@ -139,10 +141,14 @@ fn camelCaseToDashCase(name: []const u8, buf: []u8) []const u8 {
     }
 
     // Check for vendor prefixes: webkitTransform -> -webkit-transform
-    const has_vendor_prefix = name.len > 6 and (std.mem.startsWith(u8, name, "webkit") or
-        std.mem.startsWith(u8, name, "moz") or
-        std.mem.startsWith(u8, name, "ms") or
-        std.mem.startsWith(u8, name, "o"));
+    // Must have uppercase letter after the prefix
+    const has_vendor_prefix = blk: {
+        if (name.len > 6 and std.mem.startsWith(u8, name, "webkit") and std.ascii.isUpper(name[6])) break :blk true;
+        if (name.len > 3 and std.mem.startsWith(u8, name, "moz") and std.ascii.isUpper(name[3])) break :blk true;
+        if (name.len > 2 and std.mem.startsWith(u8, name, "ms") and std.ascii.isUpper(name[2])) break :blk true;
+        if (name.len > 1 and std.mem.startsWith(u8, name, "o") and std.ascii.isUpper(name[1])) break :blk true;
+        break :blk false;
+    };
 
     var write_pos: usize = 0;