diff --git a/src/browser/Factory.zig b/src/browser/Factory.zig
index 582291a3..edb6baee 100644
--- a/src/browser/Factory.zig
+++ b/src/browser/Factory.zig
@@ -270,14 +270,16 @@ pub fn abstractRange(self: *Factory, child: anytype, page: *Page) !*@TypeOf(chil
     const chain = try PrototypeChain(&.{ AbstractRange, @TypeOf(child) }).allocate(allocator);
 
     const doc = page.document.asNode();
-    chain.set(0, AbstractRange{
+    const abstract_range = chain.get(0);
+    abstract_range.* = AbstractRange{
         ._type = unionInit(AbstractRange.Type, chain.get(1)),
         ._end_offset = 0,
         ._start_offset = 0,
         ._end_container = doc,
         ._start_container = doc,
-    });
+    };
     chain.setLeaf(1, child);
+    page._live_ranges.append(&abstract_range._range_link);
     return chain.get(1);
 }
 
diff --git a/src/browser/Page.zig b/src/browser/Page.zig
index 811e7e5c..889e0d3c 100644
--- a/src/browser/Page.zig
+++ b/src/browser/Page.zig
@@ -54,6 +54,7 @@ const Performance = @import("webapi/Performance.zig");
 const Screen = @import("webapi/Screen.zig");
 const VisualViewport = @import("webapi/VisualViewport.zig");
 const PerformanceObserver = @import("webapi/PerformanceObserver.zig");
+const AbstractRange = @import("webapi/AbstractRange.zig");
 const MutationObserver = @import("webapi/MutationObserver.zig");
 const IntersectionObserver = @import("webapi/IntersectionObserver.zig");
 const CustomElementDefinition = @import("webapi/CustomElementDefinition.zig");
@@ -142,6 +143,9 @@ _to_load: std.ArrayList(*Element.Html) = .{},
 
 _script_manager: ScriptManager,
 
+// List of active live ranges (for mutation updates per DOM spec)
+_live_ranges: std.DoublyLinkedList = .{},
+
 // List of active MutationObservers
 _mutation_observers: std.DoublyLinkedList = .{},
 _mutation_delivery_scheduled: bool = false,
@@ -2394,6 +2398,12 @@ pub fn removeNode(self: *Page, parent: *Node, child: *Node, opts: RemoveNodeOpts
     const previous_sibling = child.previousSibling();
     const next_sibling = child.nextSibling();
 
+    // Capture child's index before removal for live range updates (DOM spec remove steps 4-7)
+    const child_index_for_ranges: ?u32 = if (self._live_ranges.first != null)
+        parent.getChildIndex(child)
+    else
+        null;
+
     const children = parent._children.?;
     switch (children.*) {
         .one => |n| {
@@ -2422,6 +2432,11 @@ pub fn removeNode(self: *Page, parent: *Node, child: *Node, opts: RemoveNodeOpts
     child._parent = null;
     child._child_link = .{};
 
+    // Update live ranges for removal (DOM spec remove steps 4-7)
+    if (child_index_for_ranges) |idx| {
+        self.updateRangesForNodeRemoval(parent, child, idx);
+    }
+
     // Handle slot assignment removal before mutation observers
     if (child.is(Element)) |el| {
         // Check if the parent was a shadow host
@@ -2569,6 +2584,21 @@ pub fn _insertNodeRelative(self: *Page, comptime from_parser: bool, parent: *Nod
     }
     child._parent = parent;
 
+    // Update live ranges for insertion (DOM spec insert step 6).
+    // For .before/.after the child was inserted at a specific position;
+    // ranges on parent with offsets past that position must be incremented.
+    // For .append no range update is needed (spec: "if child is non-null").
+    if (self._live_ranges.first != null) {
+        switch (relative) {
+            .append => {},
+            .before, .after => {
+                if (parent.getChildIndex(child)) |idx| {
+                    self.updateRangesForNodeInsertion(parent, idx);
+                }
+            },
+        }
+    }
+
     // Tri-state behavior for mutations:
     // 1. from_parser=true, parse_mode=document -> no mutations (initial document parse)
     // 2. from_parser=true, parse_mode=fragment -> mutations (innerHTML additions)
@@ -2827,6 +2857,54 @@ pub fn childListChange(
     }
 }
 
+// --- Live range update methods (DOM spec §4.2.3, §4.2.4, §4.7, §4.8) ---
+
+/// Update all live ranges after a replaceData mutation on a CharacterData node.
+/// Per DOM spec: insertData = replaceData(offset, 0, data),
+///               deleteData = replaceData(offset, count, "").
+/// All parameters are in UTF-16 code unit offsets.
+pub fn updateRangesForCharacterDataReplace(self: *Page, target: *Node, offset: u32, count: u32, data_len: u32) void {
+    var it: ?*std.DoublyLinkedList.Node = self._live_ranges.first;
+    while (it) |link| : (it = link.next) {
+        const ar: *AbstractRange = @fieldParentPtr("_range_link", link);
+        ar.updateForCharacterDataReplace(target, offset, count, data_len);
+    }
+}
+
+/// Update all live ranges after a splitText operation.
+/// Steps 7b-7e of the DOM spec splitText algorithm.
+/// Steps 7d-7e complement (not overlap) updateRangesForNodeInsertion:
+/// the insert update handles offsets > child_index, while 7d/7e handle
+/// offsets == node_index+1 (these are equal values but with > vs == checks).
+pub fn updateRangesForSplitText(self: *Page, target: *Node, new_node: *Node, offset: u32, parent: *Node, node_index: u32) void {
+    var it: ?*std.DoublyLinkedList.Node = self._live_ranges.first;
+    while (it) |link| : (it = link.next) {
+        const ar: *AbstractRange = @fieldParentPtr("_range_link", link);
+        ar.updateForSplitText(target, new_node, offset, parent, node_index);
+    }
+}
+
+/// Update all live ranges after a node insertion.
+/// Per DOM spec insert algorithm step 6: only applies when inserting before a
+/// non-null reference node.
+pub fn updateRangesForNodeInsertion(self: *Page, parent: *Node, child_index: u32) void {
+    var it: ?*std.DoublyLinkedList.Node = self._live_ranges.first;
+    while (it) |link| : (it = link.next) {
+        const ar: *AbstractRange = @fieldParentPtr("_range_link", link);
+        ar.updateForNodeInsertion(parent, child_index);
+    }
+}
+
+/// Update all live ranges after a node removal.
+/// Per DOM spec remove algorithm steps 4-7.
+pub fn updateRangesForNodeRemoval(self: *Page, parent: *Node, child: *Node, child_index: u32) void {
+    var it: ?*std.DoublyLinkedList.Node = self._live_ranges.first;
+    while (it) |link| : (it = link.next) {
+        const ar: *AbstractRange = @fieldParentPtr("_range_link", link);
+        ar.updateForNodeRemoval(parent, child, child_index);
+    }
+}
+
 // TODO: optimize and cleanup, this is called a lot (e.g., innerHTML = '')
 pub fn parseHtmlAsChildren(self: *Page, node: *Node, html: []const u8) !void {
     const previous_parse_mode = self._parse_mode;
diff --git a/src/browser/tests/range_mutations.html b/src/browser/tests/range_mutations.html
new file mode 100644
index 00000000..3e4efc7e
--- /dev/null
+++ b/src/browser/tests/range_mutations.html
@@ -0,0 +1,315 @@
+<!DOCTYPE html>
+<script src="testing.js"></script>
+
+<script id=insertData_adjusts_range_offsets>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 2);
+  range.setEnd(text, 5);
+  // range covers "cde"
+
+  // Insert "XX" at offset 1 (before range start)
+  text.insertData(1, 'XX');
+  // "aXXbcdef" — range should shift right by 2
+  testing.expectEqual(4, range.startOffset);
+  testing.expectEqual(7, range.endOffset);
+  testing.expectEqual(text, range.startContainer);
+}
+</script>
+
+<script id=insertData_at_range_start>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 2);
+  range.setEnd(text, 5);
+
+  // Insert at exactly the start offset — should not shift start
+  text.insertData(2, 'YY');
+  // "abYYcdef" — start stays at 2, end shifts by 2
+  testing.expectEqual(2, range.startOffset);
+  testing.expectEqual(7, range.endOffset);
+}
+</script>
+
+<script id=insertData_inside_range>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 2);
+  range.setEnd(text, 5);
+
+  // Insert inside the range
+  text.insertData(3, 'Z');
+  // "abcZdef" — start unchanged, end shifts by 1
+  testing.expectEqual(2, range.startOffset);
+  testing.expectEqual(6, range.endOffset);
+}
+</script>
+
+<script id=insertData_after_range>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 2);
+  range.setEnd(text, 5);
+
+  // Insert after range end — no change
+  text.insertData(5, 'ZZ');
+  testing.expectEqual(2, range.startOffset);
+  testing.expectEqual(5, range.endOffset);
+}
+</script>
+
+<script id=deleteData_before_range>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 3);
+  range.setEnd(text, 5);
+  // range covers "de"
+
+  // Delete "ab" (offset 0, count 2) — before range
+  text.deleteData(0, 2);
+  // "cdef" — range shifts left by 2
+  testing.expectEqual(1, range.startOffset);
+  testing.expectEqual(3, range.endOffset);
+}
+</script>
+
+<script id=deleteData_overlapping_range_start>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 2);
+  range.setEnd(text, 5);
+
+  // Delete from offset 1, count 2 — overlaps range start
+  text.deleteData(1, 2);
+  // "adef" — start clamped to offset(1), end adjusted
+  testing.expectEqual(1, range.startOffset);
+  testing.expectEqual(3, range.endOffset);
+}
+</script>
+
+<script id=deleteData_inside_range>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 1);
+  range.setEnd(text, 5);
+
+  // Delete inside range: offset 2, count 2
+  text.deleteData(2, 2);
+  // "abef" — start unchanged, end shifts by -2
+  testing.expectEqual(1, range.startOffset);
+  testing.expectEqual(3, range.endOffset);
+}
+</script>
+
+<script id=replaceData_adjusts_range>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 2);
+  range.setEnd(text, 5);
+
+  // Replace "cd" (offset 2, count 2) with "XXXX" (4 chars)
+  text.replaceData(2, 2, 'XXXX');
+  // "abXXXXef" — start clamped to 2, end adjusted by (4-2)=+2
+  testing.expectEqual(2, range.startOffset);
+  testing.expectEqual(7, range.endOffset);
+}
+</script>
+
+<script id=splitText_moves_range_to_new_node>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 4);
+  range.setEnd(text, 6);
+  // range covers "ef"
+
+  const newText = text.splitText(3);
+  // text = "abc", newText = "def"
+  // Range was at (text, 4)-(text, 6), with offset > 3:
+  // start moves to (newText, 4-3=1), end moves to (newText, 6-3=3)
+  testing.expectEqual(newText, range.startContainer);
+  testing.expectEqual(1, range.startOffset);
+  testing.expectEqual(newText, range.endContainer);
+  testing.expectEqual(3, range.endOffset);
+}
+</script>
+
+<script id=splitText_range_at_split_point>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 0);
+  range.setEnd(text, 3);
+  // range covers "abc"
+
+  const newText = text.splitText(3);
+  // text = "abc", newText = "def"
+  // Range end is at exactly the split offset — should stay on original node
+  testing.expectEqual(text, range.startContainer);
+  testing.expectEqual(0, range.startOffset);
+  testing.expectEqual(text, range.endContainer);
+  testing.expectEqual(3, range.endOffset);
+}
+</script>
+
+<script id=appendChild_does_not_affect_range>
+{
+  const div = document.createElement('div');
+  const p1 = document.createElement('p');
+  const p2 = document.createElement('p');
+  div.appendChild(p1);
+  div.appendChild(p2);
+
+  const range = document.createRange();
+  range.setStart(div, 0);
+  range.setEnd(div, 2);
+
+  // Appending should not affect range offsets (spec: no update for append)
+  const p3 = document.createElement('p');
+  div.appendChild(p3);
+  testing.expectEqual(0, range.startOffset);
+  testing.expectEqual(2, range.endOffset);
+}
+</script>
+
+<script id=insertBefore_shifts_range_offsets>
+{
+  const div = document.createElement('div');
+  const p1 = document.createElement('p');
+  const p2 = document.createElement('p');
+  div.appendChild(p1);
+  div.appendChild(p2);
+
+  const range = document.createRange();
+  range.setStart(div, 1);
+  range.setEnd(div, 2);
+
+  // Insert before p1 (index 0) — range offsets > 0 should increment
+  const span = document.createElement('span');
+  div.insertBefore(span, p1);
+  testing.expectEqual(2, range.startOffset);
+  testing.expectEqual(3, range.endOffset);
+}
+</script>
+
+<script id=removeChild_shifts_range_offsets>
+{
+  const div = document.createElement('div');
+  const p1 = document.createElement('p');
+  const p2 = document.createElement('p');
+  const p3 = document.createElement('p');
+  div.appendChild(p1);
+  div.appendChild(p2);
+  div.appendChild(p3);
+
+  const range = document.createRange();
+  range.setStart(div, 1);
+  range.setEnd(div, 3);
+
+  // Remove p1 (index 0) — offsets > 0 should decrement
+  div.removeChild(p1);
+  testing.expectEqual(0, range.startOffset);
+  testing.expectEqual(2, range.endOffset);
+}
+</script>
+
+<script id=removeChild_moves_range_from_descendant>
+{
+  const div = document.createElement('div');
+  const p = document.createElement('p');
+  const text = document.createTextNode('hello');
+  p.appendChild(text);
+  div.appendChild(p);
+
+  const range = document.createRange();
+  range.setStart(text, 2);
+  range.setEnd(text, 4);
+
+  // Remove p (which contains text) — range should move to (div, index_of_p)
+  div.removeChild(p);
+  testing.expectEqual(div, range.startContainer);
+  testing.expectEqual(0, range.startOffset);
+  testing.expectEqual(div, range.endContainer);
+  testing.expectEqual(0, range.endOffset);
+}
+</script>
+
+<script id=multiple_ranges_updated>
+{
+  const text = document.createTextNode('abcdefgh');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range1 = document.createRange();
+  range1.setStart(text, 1);
+  range1.setEnd(text, 3);
+
+  const range2 = document.createRange();
+  range2.setStart(text, 5);
+  range2.setEnd(text, 7);
+
+  // Insert at offset 0 — both ranges should shift
+  text.insertData(0, 'XX');
+  testing.expectEqual(3, range1.startOffset);
+  testing.expectEqual(5, range1.endOffset);
+  testing.expectEqual(7, range2.startOffset);
+  testing.expectEqual(9, range2.endOffset);
+}
+</script>
+
+<script id=data_setter_updates_ranges>
+{
+  const text = document.createTextNode('abcdef');
+  const div = document.createElement('div');
+  div.appendChild(text);
+
+  const range = document.createRange();
+  range.setStart(text, 2);
+  range.setEnd(text, 5);
+
+  // Setting data replaces all content — range collapses to offset 0
+  text.data = 'new content';
+  testing.expectEqual(text, range.startContainer);
+  testing.expectEqual(0, range.startOffset);
+  testing.expectEqual(text, range.endContainer);
+  testing.expectEqual(0, range.endOffset);
+}
+</script>
diff --git a/src/browser/webapi/AbstractRange.zig b/src/browser/webapi/AbstractRange.zig
index 5f3edc31..e766ac29 100644
--- a/src/browser/webapi/AbstractRange.zig
+++ b/src/browser/webapi/AbstractRange.zig
@@ -33,6 +33,9 @@ _start_offset: u32,
 _end_container: *Node,
 _start_container: *Node,
 
+// Intrusive linked list node for tracking live ranges on the Page.
+_range_link: std.DoublyLinkedList.Node = .{},
+
 pub const Type = union(enum) {
     range: *Range,
     // TODO: static_range: *StaticRange,
@@ -215,6 +218,91 @@ fn isInclusiveAncestorOf(potential_ancestor: *Node, node: *Node) bool {
     return isAncestorOf(potential_ancestor, node);
 }
 
+/// Update this range's boundaries after a replaceData mutation on target.
+/// All parameters are in UTF-16 code unit offsets.
+pub fn updateForCharacterDataReplace(self: *AbstractRange, target: *Node, offset: u32, count: u32, data_len: u32) void {
+    if (self._start_container == target) {
+        if (self._start_offset > offset and self._start_offset <= offset + count) {
+            self._start_offset = offset;
+        } else if (self._start_offset > offset + count) {
+            // Use i64 intermediate to avoid u32 underflow when count > data_len
+            self._start_offset = @intCast(@as(i64, self._start_offset) + @as(i64, data_len) - @as(i64, count));
+        }
+    }
+
+    if (self._end_container == target) {
+        if (self._end_offset > offset and self._end_offset <= offset + count) {
+            self._end_offset = offset;
+        } else if (self._end_offset > offset + count) {
+            self._end_offset = @intCast(@as(i64, self._end_offset) + @as(i64, data_len) - @as(i64, count));
+        }
+    }
+}
+
+/// Update this range's boundaries after a splitText operation.
+/// Steps 7b-7e of the DOM spec splitText algorithm.
+pub fn updateForSplitText(self: *AbstractRange, target: *Node, new_node: *Node, offset: u32, parent: *Node, node_index: u32) void {
+    // Step 7b: ranges on the original node with start > offset move to new node
+    if (self._start_container == target and self._start_offset > offset) {
+        self._start_container = new_node;
+        self._start_offset = self._start_offset - offset;
+    }
+    // Step 7c: ranges on the original node with end > offset move to new node
+    if (self._end_container == target and self._end_offset > offset) {
+        self._end_container = new_node;
+        self._end_offset = self._end_offset - offset;
+    }
+    // Step 7d: ranges on parent with start == node_index + 1 increment
+    if (self._start_container == parent and self._start_offset == node_index + 1) {
+        self._start_offset += 1;
+    }
+    // Step 7e: ranges on parent with end == node_index + 1 increment
+    if (self._end_container == parent and self._end_offset == node_index + 1) {
+        self._end_offset += 1;
+    }
+}
+
+/// Update this range's boundaries after a node insertion.
+pub fn updateForNodeInsertion(self: *AbstractRange, parent: *Node, child_index: u32) void {
+    if (self._start_container == parent and self._start_offset > child_index) {
+        self._start_offset += 1;
+    }
+    if (self._end_container == parent and self._end_offset > child_index) {
+        self._end_offset += 1;
+    }
+}
+
+/// Update this range's boundaries after a node removal.
+pub fn updateForNodeRemoval(self: *AbstractRange, parent: *Node, child: *Node, child_index: u32) void {
+    // Steps 4-5: ranges whose start/end is an inclusive descendant of child
+    // get moved to (parent, child_index).
+    if (isInclusiveDescendantOf(self._start_container, child)) {
+        self._start_container = parent;
+        self._start_offset = child_index;
+    }
+    if (isInclusiveDescendantOf(self._end_container, child)) {
+        self._end_container = parent;
+        self._end_offset = child_index;
+    }
+
+    // Steps 6-7: ranges on parent at offsets > child_index get decremented.
+    if (self._start_container == parent and self._start_offset > child_index) {
+        self._start_offset -= 1;
+    }
+    if (self._end_container == parent and self._end_offset > child_index) {
+        self._end_offset -= 1;
+    }
+}
+
+fn isInclusiveDescendantOf(node: *Node, potential_ancestor: *Node) bool {
+    var current: ?*Node = node;
+    while (current) |n| {
+        if (n == potential_ancestor) return true;
+        current = n.parentNode();
+    }
+    return false;
+}
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(AbstractRange);
 
diff --git a/src/browser/webapi/CData.zig b/src/browser/webapi/CData.zig
index 13e075ad..4fb6de6f 100644
--- a/src/browser/webapi/CData.zig
+++ b/src/browser/webapi/CData.zig
@@ -37,7 +37,7 @@ _data: String = .empty,
 /// Count UTF-16 code units in a UTF-8 string.
 /// 4-byte UTF-8 sequences (codepoints >= U+10000) produce 2 UTF-16 code units (surrogate pair),
 /// everything else produces 1.
-fn utf16Len(data: []const u8) usize {
+pub fn utf16Len(data: []const u8) usize {
     var count: usize = 0;
     var i: usize = 0;
     while (i < data.len) {
@@ -232,14 +232,13 @@ pub fn setData(self: *CData, value: ?[]const u8, page: *Page) !void {
 }
 
 /// JS bridge wrapper for `data` setter.
-/// Handles [LegacyNullToEmptyString]: null → setData(null) → "".
-/// Passes everything else (including undefined) through V8 toString,
-/// so `undefined` becomes the string "undefined" per spec.
+/// Per spec, setting .data runs replaceData(0, this.length, value),
+/// which includes live range updates.
+/// Handles [LegacyNullToEmptyString]: null → "" per spec.
 pub fn _setData(self: *CData, value: js.Value, page: *Page) !void {
-    if (value.isNull()) {
-        return self.setData(null, page);
-    }
-    return self.setData(try value.toZig([]const u8), page);
+    const new_value: []const u8 = if (value.isNull()) "" else try value.toZig([]const u8);
+    const length = self.getLength();
+    try self.replaceData(0, length, new_value, page);
 }
 
 pub fn format(self: *const CData, writer: *std.io.Writer) !void {
@@ -272,15 +271,20 @@ pub fn isEqualNode(self: *const CData, other: *const CData) bool {
 }
 
 pub fn appendData(self: *CData, data: []const u8, page: *Page) !void {
-    const old_value = self._data;
-    self._data = try String.concat(page.arena, &.{ self._data.str(), data });
-    page.characterDataChange(self.asNode(), old_value);
+    // Per DOM spec, appendData(data) is replaceData(length, 0, data).
+    const length = self.getLength();
+    try self.replaceData(length, 0, data, page);
 }
 
 pub fn deleteData(self: *CData, offset: usize, count: usize, page: *Page) !void {
     const end_utf16 = std.math.add(usize, offset, count) catch std.math.maxInt(usize);
     const range = try utf16RangeToUtf8(self._data.str(), offset, end_utf16);
 
+    // Update live ranges per DOM spec replaceData steps (deleteData = replaceData with data="")
+    const length = self.getLength();
+    const effective_count: u32 = @intCast(@min(count, length - offset));
+    page.updateRangesForCharacterDataReplace(self.asNode(), @intCast(offset), effective_count, 0);
+
     const old_data = self._data;
     const old_value = old_data.str();
     if (range.start == 0) {
@@ -299,6 +303,10 @@ pub fn deleteData(self: *CData, offset: usize, count: usize, page: *Page) !void
 
 pub fn insertData(self: *CData, offset: usize, data: []const u8, page: *Page) !void {
     const byte_offset = try utf16OffsetToUtf8(self._data.str(), offset);
+
+    // Update live ranges per DOM spec replaceData steps (insertData = replaceData with count=0)
+    page.updateRangesForCharacterDataReplace(self.asNode(), @intCast(offset), 0, @intCast(utf16Len(data)));
+
     const old_value = self._data;
     const existing = old_value.str();
     self._data = try String.concat(page.arena, &.{
@@ -312,6 +320,12 @@ pub fn insertData(self: *CData, offset: usize, data: []const u8, page: *Page) !v
 pub fn replaceData(self: *CData, offset: usize, count: usize, data: []const u8, page: *Page) !void {
     const end_utf16 = std.math.add(usize, offset, count) catch std.math.maxInt(usize);
     const range = try utf16RangeToUtf8(self._data.str(), offset, end_utf16);
+
+    // Update live ranges per DOM spec replaceData steps
+    const length = self.getLength();
+    const effective_count: u32 = @intCast(@min(count, length - offset));
+    page.updateRangesForCharacterDataReplace(self.asNode(), @intCast(offset), effective_count, @intCast(utf16Len(data)));
+
     const old_value = self._data;
     const existing = old_value.str();
     self._data = try String.concat(page.arena, &.{
diff --git a/src/browser/webapi/Node.zig b/src/browser/webapi/Node.zig
index 15541491..7bfa7cca 100644
--- a/src/browser/webapi/Node.zig
+++ b/src/browser/webapi/Node.zig
@@ -293,7 +293,8 @@ pub fn setTextContent(self: *Node, data: []const u8, page: *Page) !void {
             }
             return el.replaceChildren(&.{.{ .text = data }}, page);
         },
-        .cdata => |c| c._data = try page.dupeSSO(data),
+        // Per spec, setting textContent on CharacterData runs replaceData(0, length, value)
+        .cdata => |c| try c.replaceData(0, c.getLength(), data, page),
         .document => {},
         .document_type => {},
         .document_fragment => |frag| {
@@ -612,7 +613,11 @@ pub fn getNodeValue(self: *const Node) ?String {
 
 pub fn setNodeValue(self: *const Node, value: ?String, page: *Page) !void {
     switch (self._type) {
-        .cdata => |c| try c.setData(if (value) |v| v.str() else null, page),
+        // Per spec, setting nodeValue on CharacterData runs replaceData(0, length, value)
+        .cdata => |c| {
+            const new_value: []const u8 = if (value) |v| v.str() else "";
+            try c.replaceData(0, c.getLength(), new_value, page);
+        },
         .attribute => |attr| try attr.setValue(value, page),
         .element => {},
         .document => {},
diff --git a/src/browser/webapi/Range.zig b/src/browser/webapi/Range.zig
index f0c904a6..66ce1c93 100644
--- a/src/browser/webapi/Range.zig
+++ b/src/browser/webapi/Range.zig
@@ -322,6 +322,11 @@ pub fn insertNode(self: *Range, node: *Node, page: *Page) !void {
     const container = self._proto._start_container;
     const offset = self._proto._start_offset;
 
+    // Per spec: if range is collapsed, end offset should extend to include
+    // the inserted node. Capture before insertion since live range updates
+    // in the insert path will adjust non-collapsed ranges automatically.
+    const was_collapsed = self._proto.getCollapsed();
+
     if (container.is(Node.CData)) |_| {
         // If container is a text node, we need to split it
         const parent = container.parentNode() orelse return error.InvalidNodeType;
@@ -351,9 +356,10 @@ pub fn insertNode(self: *Range, node: *Node, page: *Page) !void {
         _ = try container.insertBefore(node, ref_child, page);
     }
 
-    // Update range to be after the inserted node
-    if (self._proto._start_container == self._proto._end_container) {
-        self._proto._end_offset += 1;
+    // Per spec step 11: if range was collapsed, extend end to include inserted node.
+    // Non-collapsed ranges are already handled by the live range update in the insert path.
+    if (was_collapsed) {
+        self._proto._end_offset = self._proto._start_offset + 1;
     }
 }
 
@@ -375,9 +381,12 @@ pub fn deleteContents(self: *Range, page: *Page) !void {
             );
             page.characterDataChange(self._proto._start_container, old_value);
         } else {
-            // Delete child nodes in range
-            var offset = self._proto._start_offset;
-            while (offset < self._proto._end_offset) : (offset += 1) {
+            // Delete child nodes in range.
+            // Capture count before the loop: removeChild triggers live range
+            // updates that decrement _end_offset on each removal.
+            const count = self._proto._end_offset - self._proto._start_offset;
+            var i: u32 = 0;
+            while (i < count) : (i += 1) {
                 if (self._proto._start_container.getChildAt(self._proto._start_offset)) |child| {
                     _ = try self._proto._start_container.removeChild(child, page);
                 }
@@ -717,3 +726,6 @@ const testing = @import("../../testing.zig");
 test "WebApi: Range" {
     try testing.htmlRunner("range.html", .{});
 }
+test "WebApi: Range mutations" {
+    try testing.htmlRunner("range_mutations.html", .{});
+}
diff --git a/src/browser/webapi/cdata/Text.zig b/src/browser/webapi/cdata/Text.zig
index 5eb096f3..e7a338b5 100644
--- a/src/browser/webapi/cdata/Text.zig
+++ b/src/browser/webapi/cdata/Text.zig
@@ -43,16 +43,26 @@ pub fn splitText(self: *Text, offset: usize, page: *Page) !*Text {
     const new_node = try page.createTextNode(new_data);
     const new_text = new_node.as(Text);
 
-    const old_data = data[0..byte_offset];
-    try self._proto.setData(old_data, page);
-
-    // If this node has a parent, insert the new node right after this one
     const node = self._proto.asNode();
+
+    // Per DOM spec splitText: insert first (step 7a), then update ranges (7b-7e),
+    // then truncate original node (step 8).
     if (node.parentNode()) |parent| {
         const next_sibling = node.nextSibling();
         _ = try parent.insertBefore(new_node, next_sibling, page);
+
+        // splitText-specific range updates (steps 7b-7e)
+        if (parent.getChildIndex(node)) |node_index| {
+            page.updateRangesForSplitText(node, new_node, @intCast(offset), parent, node_index);
+        }
     }
 
+    // Step 8: truncate original node via replaceData(offset, count, "").
+    // Use replaceData instead of setData so live range updates fire
+    // (matters for detached text nodes where steps 7b-7e were skipped).
+    const length = self._proto.getLength();
+    try self._proto.replaceData(offset, length - offset, "", page);
+
     return new_text;
 }