diff --git a/src/browser/tests/element/css_style_properties.html b/src/browser/tests/element/css_style_properties.html
index 6cb0e0bdc..3592df67e 100644
--- a/src/browser/tests/element/css_style_properties.html
+++ b/src/browser/tests/element/css_style_properties.html
@@ -159,6 +159,35 @@
 <div id=crash1 style="background-position: 5% .1em"></div>
 <script id="crash_case_1">
 {
-  testing.expectEqual('5% .1em', $('#crash1').style.backgroundPosition);
+  // Per CSSOM, bare ".1em" serializes with a leading zero -> "0.1em"
+  testing.expectEqual('5% 0.1em', $('#crash1').style.backgroundPosition);
+}
+</script>
+
+<div id=opacity-detect style="opacity:.55"></div>
+<script id="leadingZeroNormalization">
+{
+  // jQuery's support.opacity feature-detect reads an inline "opacity:.55"
+  // back and matches it against /^0.55$/; bare ".55" must serialize as "0.55".
+  testing.expectEqual('0.55', $('#opacity-detect').style.opacity);
+
+  const div = $('#test-div');
+  div.style.cssText = '';
+
+  // leading-dot value via property assignment
+  div.style.opacity = '.5';
+  testing.expectEqual('0.5', div.style.opacity);
+
+  // negative leading-dot, with unit, preserved otherwise
+  div.style.marginLeft = '-.5px';
+  testing.expectEqual('-0.5px', div.style.marginLeft);
+
+  // existing leading digit is untouched
+  div.style.lineHeight = '1.5';
+  testing.expectEqual('1.5', div.style.lineHeight);
+
+  // multiple numbers in one value
+  div.style.margin = '.5em .25em';
+  testing.expectEqual('0.5em 0.25em', div.style.margin);
 }
 </script>
diff --git a/src/browser/webapi/css/CSSStyleDeclaration.zig b/src/browser/webapi/css/CSSStyleDeclaration.zig
index 12833c412..f1c915506 100644
--- a/src/browser/webapi/css/CSSStyleDeclaration.zig
+++ b/src/browser/webapi/css/CSSStyleDeclaration.zig
@@ -245,7 +245,9 @@ fn normalizePropertyName(name: []const u8, buf: []u8) []const u8 {
 }
 
 // Normalize CSS property values for canonical serialization
-fn normalizePropertyValue(arena: Allocator, property_name: []const u8, value: []const u8) ![]const u8 {
+fn normalizePropertyValue(arena: Allocator, property_name: []const u8, raw_value: []const u8) ![]const u8 {
+    const value = try normalizeLeadingZeros(arena, raw_value);
+
     // Per CSSOM spec, unitless zero in length properties should serialize as "0px"
     if (std.mem.eql(u8, value, "0") and isLengthProperty(property_name)) {
         return "0px";
@@ -286,6 +288,51 @@ fn normalizePropertyValue(arena: Allocator, property_name: []const u8, value: []
     return value;
 }
 
+// Insert a leading "0" before any bare ".<digit>". A value might have multiple
+// such bare digits
+fn normalizeLeadingZeros(arena: Allocator, value: []const u8) ![]const u8 {
+    // A bare ".<digit>" needs at least 2 chars, and a trailing "." can never be
+    // followed by a digit. So we only ever scan up to value.len - 1, which lets
+    // us index value[i + 1] without a bounds check.
+    if (value.len < 2) {
+        return value;
+    }
+
+    var inserts: usize = 0;
+    for (value[0 .. value.len - 1], 0..) |c, i| {
+        if (c != '.') {
+            continue;
+        }
+        if (!std.ascii.isDigit(value[i + 1])) {
+            // next value isn't a digit
+            continue;
+        }
+        if (i > 0 and std.ascii.isDigit(value[i - 1])) {
+            // previous value is a digit
+            continue;
+        }
+        inserts += 1;
+    }
+    if (inserts == 0) {
+        return value;
+    }
+
+    const buf = try arena.alloc(u8, value.len + inserts);
+    var w: usize = 0;
+    for (value[0 .. value.len - 1], 0..) |c, i| {
+        if (c == '.' and std.ascii.isDigit(value[i + 1]) and (i == 0 or !std.ascii.isDigit(value[i - 1]))) {
+            buf[w] = '0';
+            w += 1;
+        }
+        buf[w] = c;
+        w += 1;
+    }
+    // The last char is copied unconditionally: it can't start a bare ".<digit>".
+    buf[w] = value[value.len - 1];
+    w += 1;
+    return buf[0..w];
+}
+
 // Canonicalize anchor-size() so that the dashed ident (anchor name) comes before the size keyword.
 // e.g. "anchor-size(width --foo)" -> "anchor-size(--foo width)"
 fn canonicalizeAnchorSize(arena: Allocator, value: []const u8, start_index: usize) ![]const u8 {