diff --git a/src/browser/css/Parser.zig b/src/browser/css/Parser.zig
index 34309631..526aa1db 100644
--- a/src/browser/css/Parser.zig
+++ b/src/browser/css/Parser.zig
@@ -293,3 +293,143 @@ fn isBang(token: Tokenizer.Token) bool {
         else => false,
     };
 }
+
+pub const Rule = struct {
+    selector: []const u8,
+    block: []const u8,
+};
+
+pub fn parseStylesheet(input: []const u8) RulesIterator {
+    return RulesIterator.init(input);
+}
+
+pub const RulesIterator = struct {
+    input: []const u8,
+    stream: TokenStream,
+
+    pub fn init(input: []const u8) RulesIterator {
+        return .{
+            .input = input,
+            .stream = TokenStream.init(input),
+        };
+    }
+
+    pub fn next(self: *RulesIterator) ?Rule {
+        var selector_start: ?usize = null;
+        var selector_end: ?usize = null;
+
+        // Skip leading trivia
+        while (self.stream.peek()) |peeked| {
+            if (!isWhitespaceOrComment(peeked.token)) break;
+            _ = self.stream.next();
+        }
+
+        while (true) {
+            const peeked = self.stream.peek() orelse return null;
+
+            if (isCurlyBlockStart(peeked.token)) {
+                if (selector_start == null) {
+                    self.skipBlock();
+                    continue;
+                }
+
+                const open_brace = self.stream.next() orelse return null;
+                const block_start = open_brace.end;
+                var block_end = block_start;
+
+                var depth: usize = 1;
+                while (true) {
+                    const span = self.stream.next() orelse {
+                        block_end = self.input.len;
+                        break;
+                    };
+                    if (isCurlyBlockStart(span.token)) {
+                        depth += 1;
+                    } else if (isCurlyBlockEnd(span.token)) {
+                        depth -= 1;
+                        if (depth == 0) {
+                            block_end = span.start;
+                            break;
+                        }
+                    }
+                }
+
+                var selector = self.input[selector_start.?..selector_end.?];
+                selector = std.mem.trim(u8, selector, &std.ascii.whitespace);
+
+                return .{
+                    .selector = selector,
+                    .block = self.input[block_start..block_end],
+                };
+            }
+
+            if (peeked.token == .at_keyword) {
+                self.skipAtRule();
+                selector_start = null;
+                selector_end = null;
+                continue;
+            }
+
+            const span = self.stream.next() orelse return null;
+            if (!isWhitespaceOrComment(span.token)) {
+                if (selector_start == null) selector_start = span.start;
+                selector_end = span.end;
+            }
+        }
+    }
+
+    fn skipBlock(self: *RulesIterator) void {
+        const span = self.stream.next() orelse return;
+        if (!isCurlyBlockStart(span.token)) return;
+
+        var depth: usize = 1;
+        while (true) {
+            const next_span = self.stream.next() orelse return;
+            if (isCurlyBlockStart(next_span.token)) {
+                depth += 1;
+            } else if (isCurlyBlockEnd(next_span.token)) {
+                depth -= 1;
+                if (depth == 0) return;
+            }
+        }
+    }
+
+    fn skipAtRule(self: *RulesIterator) void {
+        _ = self.stream.next(); // consume @keyword
+        var depth: usize = 0;
+        var saw_block = false;
+
+        while (true) {
+            const peeked = self.stream.peek() orelse return;
+            if (!saw_block and isSemicolon(peeked.token) and depth == 0) {
+                _ = self.stream.next();
+                return;
+            }
+
+            const span = self.stream.next() orelse return;
+            if (isWhitespaceOrComment(span.token)) continue;
+
+            if (isCurlyBlockStart(span.token)) {
+                depth += 1;
+                saw_block = true;
+            } else if (isCurlyBlockEnd(span.token)) {
+                if (depth > 0) depth -= 1;
+                if (saw_block and depth == 0) return;
+            }
+        }
+    }
+};
+
+fn isCurlyBlockStart(token: Tokenizer.Token) bool {
+    return switch (token) {
+        .curly_bracket_block => true,
+        else => false,
+    };
+}
+
+fn isCurlyBlockEnd(token: Tokenizer.Token) bool {
+    return switch (token) {
+        .close_curly_bracket => true,
+        else => false,
+    };
+}
diff --git a/src/browser/webapi/css/CSSRuleList.zig b/src/browser/webapi/css/CSSRuleList.zig
index 7e727a56..7f9d1a77 100644
--- a/src/browser/webapi/css/CSSRuleList.zig
+++ b/src/browser/webapi/css/CSSRuleList.zig
@@ -5,21 +5,39 @@ const CSSRule = @import("CSSRule.zig");
 
 const CSSRuleList = @This();
 
-_rules: []*CSSRule = &.{},
+_rules: std.ArrayListUnmanaged(*CSSRule) = .{},
 
 pub fn init(page: *Page) !*CSSRuleList {
     return page._factory.create(CSSRuleList{});
 }
 
 pub fn length(self: *const CSSRuleList) u32 {
-    return @intCast(self._rules.len);
+    return @intCast(self._rules.items.len);
 }
 
 pub fn item(self: *const CSSRuleList, index: usize) ?*CSSRule {
-    if (index >= self._rules.len) {
+    if (index >= self._rules.items.len) {
         return null;
     }
-    return self._rules[index];
+    return self._rules.items[index];
+}
+
+pub fn insert(self: *CSSRuleList, index: u32, rule: *CSSRule, page: *Page) !void {
+    if (index > self._rules.items.len) {
+        return error.IndexSizeError; // Or standard DOMException mapped error
+    }
+    try self._rules.insert(page.arena, index, rule);
+}
+
+pub fn remove(self: *CSSRuleList, index: u32) void {
+    if (index >= self._rules.items.len) {
+        return; // Ignore or throw? Standard says IndexSizeError DOMException, but we might just no-op or return an error depending on the caller.
+    }
+    _ = self._rules.orderedRemove(index);
+}
+
+pub fn clear(self: *CSSRuleList) void {
+    self._rules.clearRetainingCapacity();
 }
 
 pub const JsApi = struct {
diff --git a/src/browser/webapi/css/CSSStyleRule.zig b/src/browser/webapi/css/CSSStyleRule.zig
index cff5ebae..767ae46e 100644
--- a/src/browser/webapi/css/CSSStyleRule.zig
+++ b/src/browser/webapi/css/CSSStyleRule.zig
@@ -34,6 +34,21 @@ pub fn getStyle(self: *CSSStyleRule, page: *Page) !*CSSStyleDeclaration {
     return style;
 }
 
+pub fn getCssText(self: *CSSStyleRule, page: *Page) ![]const u8 {
+    const style = try self.getStyle(page);
+    var buf = std.Io.Writer.Allocating.init(page.call_arena);
+    try buf.writer.print("{s} {{ ", .{self._selector_text});
+    try style.format(&buf.writer);
+    try buf.writer.writeAll(" }");
+    return buf.written();
+}
+
+pub fn setCssText(self: *CSSStyleRule, text: []const u8, page: *Page) !void {
+    _ = self;
+    _ = text;
+    _ = page;
+}
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(CSSStyleRule);
 
@@ -45,4 +60,5 @@ pub const JsApi = struct {
 
     pub const selectorText = bridge.accessor(CSSStyleRule.getSelectorText, CSSStyleRule.setSelectorText, .{});
     pub const style = bridge.accessor(CSSStyleRule.getStyle, null, .{});
+    pub const cssText = bridge.accessor(CSSStyleRule.getCssText, CSSStyleRule.setCssText, .{});
 };
diff --git a/src/browser/webapi/css/CSSStyleSheet.zig b/src/browser/webapi/css/CSSStyleSheet.zig
index 5500f63c..673ff370 100644
--- a/src/browser/webapi/css/CSSStyleSheet.zig
+++ b/src/browser/webapi/css/CSSStyleSheet.zig
@@ -4,6 +4,8 @@ const Page = @import("../../Page.zig");
 const Element = @import("../Element.zig");
 const CSSRuleList = @import("CSSRuleList.zig");
 const CSSRule = @import("CSSRule.zig");
+const CSSStyleRule = @import("CSSStyleRule.zig");
+const Parser = @import("../../css/Parser.zig");
 
 const CSSStyleSheet = @This();
 
@@ -54,30 +56,46 @@ pub fn getOwnerRule(self: *const CSSStyleSheet) ?*CSSRule {
 }
 
 pub fn insertRule(self: *CSSStyleSheet, rule: []const u8, index: u32, page: *Page) !u32 {
-    _ = self;
-    _ = rule;
-    _ = index;
-    _ = page;
-    return 0;
+    var it = Parser.parseStylesheet(rule);
+    const parsed_rule = it.next() orelse return error.SyntaxError;
+
+    const style_rule = try CSSStyleRule.init(page);
+    try style_rule.setSelectorText(parsed_rule.selector, page);
+
+    const style = try style_rule.getStyle(page);
+    try style.setCssText(parsed_rule.block, page);
+
+    const rules = try self.getCssRules(page);
+    try rules.insert(index, style_rule._proto, page);
+    return index;
 }
 
 pub fn deleteRule(self: *CSSStyleSheet, index: u32, page: *Page) !void {
-    _ = self;
-    _ = index;
-    _ = page;
+    const rules = try self.getCssRules(page);
+    rules.remove(index);
 }
 
 pub fn replace(self: *CSSStyleSheet, text: []const u8, page: *Page) !js.Promise {
-    _ = self;
-    _ = text;
-    // TODO: clear self.css_rules
+    try self.replaceSync(text, page);
     return page.js.local.?.resolvePromise({});
 }
 
-pub fn replaceSync(self: *CSSStyleSheet, text: []const u8) !void {
-    _ = self;
-    _ = text;
-    // TODO: clear self.css_rules
+pub fn replaceSync(self: *CSSStyleSheet, text: []const u8, page: *Page) !void {
+    const rules = try self.getCssRules(page);
+    rules.clear();
+
+    var it = Parser.parseStylesheet(text);
+    var index: u32 = 0;
+    while (it.next()) |parsed_rule| {
+        const style_rule = try CSSStyleRule.init(page);
+        try style_rule.setSelectorText(parsed_rule.selector, page);
+
+        const style = try style_rule.getStyle(page);
+        try style.setCssText(parsed_rule.block, page);
+
+        try rules.insert(index, style_rule._proto, page);
+        index += 1;
+    }
 }
 
 pub const JsApi = struct {
diff --git a/src/browser/webapi/element/html/Style.zig b/src/browser/webapi/element/html/Style.zig
index 131b7634..3a5f4bc5 100644
--- a/src/browser/webapi/element/html/Style.zig
+++ b/src/browser/webapi/element/html/Style.zig
@@ -94,6 +94,10 @@ pub fn getSheet(self: *Style, page: *Page) !?*CSSStyleSheet {
     if (self._sheet) |sheet| return sheet;
     const sheet = try CSSStyleSheet.initWithOwner(self.asElement(), page);
     self._sheet = sheet;
+
+    const text = try self.asNode().getTextContentAlloc(page.call_arena);
+    try sheet.replaceSync(text, page);
+
     return sheet;
 }