webapi: remove "en" lang fallback

The "en" css fallback on document. I tested a number of pages, and pages with
no lang attribute return null on a `:lang('en')` query.

src/browser/tests/element/pseudo_classes.html

@@ -99,3 +99,33 @@
     testing.expectEqual('escaped', escaped.id);
 }
 </script>
+
+<script id="lang">
+{
+    // No lang attribute anywhere: nothing has a language, so nothing matches.
+    testing.expectEqual(0, document.querySelectorAll(':lang(en)').length);
+
+    const outer = document.createElement('div');
+    outer.innerHTML = '<p lang="en-AU"><span id="inherits"></span><span id="fr" lang="fr"></span><span id="unknown" lang=""></span></p><i id="none"></i>';
+    const p = outer.querySelector('p');
+
+    testing.expectTrue(p.matches(':lang(en)'));
+    testing.expectTrue(p.matches(':lang(en-au)'));
+    testing.expectFalse(p.matches(':lang(en-US)'));
+    testing.expectFalse(p.matches(':lang(e)'));
+
+    testing.expectTrue(outer.querySelector('#inherits').matches(':lang(en)'));
+
+    // The nearest lang attribute wins, and an empty one means unknown.
+    testing.expectFalse(outer.querySelector('#fr').matches(':lang(en)'));
+    testing.expectTrue(outer.querySelector('#fr').matches(':lang(fr)'));
+    testing.expectFalse(outer.querySelector('#unknown').matches(':lang(en)'));
+
+    testing.expectFalse(outer.querySelector('#none').matches(':lang(en)'));
+    testing.expectEqual(2, outer.querySelectorAll(':lang(en)').length);
+
+    document.body.appendChild(outer);
+    testing.expectEqual(2, document.querySelectorAll(':lang(en)').length);
+    outer.remove();
+}
+</script>

src/browser/webapi/selector/List.zig

@@ -659,10 +659,7 @@ fn matchesPseudoClass(el: *Node.Element, pseudo: Selector.PseudoClass, scope: *N
         // Functional
         .lang => |expected| {
             if (expected.len == 0) return false;
-            // The element's language is the nearest ancestor-or-self lang
-            // attribute. Elements in a document with no declared language
-            // fall back to the UA default (en); detached subtrees have no
-            // language at all.
+            // The element's language is the nearest ancestor-or-self lang attribute.
             const lang = blk: {
                 var current: ?*Node = node;
                 while (current) |cur| : (current = cur.parentNode()) {
@@ -672,9 +669,6 @@ fn matchesPseudoClass(el: *Node.Element, pseudo: Selector.PseudoClass, scope: *N
                                 break :blk value;
                             }
                         },
-                        .document => {
-                            break :blk "en";
-                        },
                         else => {},
                     }
                 }