fix tabIndex default for interactive elements per spec

interactive elements (input, button, a, select, textarea, iframe)
default to 0 when tabindex attribute is absent; others default to -1.
also add TODO for hidden "until-found" tristate.

src/browser/tests/element/html/htmlelement-props.html

@@ -30,10 +30,27 @@
     i1.tabIndex = 10;
     testing.expectEqual(10, i1.tabIndex);
 
+    // Non-interactive elements default to -1
     const d3 = document.getElementById('d3');
     testing.expectEqual(-1, d3.tabIndex);
 
     d3.tabIndex = 0;
     testing.expectEqual(0, d3.tabIndex);
+
+    // Interactive elements default to 0 per spec
+    const input = document.createElement('input');
+    testing.expectEqual(0, input.tabIndex);
+
+    const button = document.createElement('button');
+    testing.expectEqual(0, button.tabIndex);
+
+    const a = document.createElement('a');
+    testing.expectEqual(0, a.tabIndex);
+
+    const select = document.createElement('select');
+    testing.expectEqual(0, select.tabIndex);
+
+    const textarea = document.createElement('textarea');
+    testing.expectEqual(0, textarea.tabIndex);
   }
 </script>

src/browser/webapi/element/Html.zig

@@ -342,6 +342,8 @@ pub fn click(self: *HtmlElement, page: *Page) !void {
     try page._event_manager.dispatch(self.asEventTarget(), event);
 }
 
+// TODO: Per spec, hidden is a tristate: true | false | "until-found".
+// We only support boolean for now; "until-found" would need bridge union support.
 pub fn getHidden(self: *HtmlElement) bool {
     return self.asElement().getAttributeSafe(comptime .wrap("hidden")) != null;
 }
@@ -355,7 +357,13 @@ pub fn setHidden(self: *HtmlElement, hidden: bool, page: *Page) !void {
 }
 
 pub fn getTabIndex(self: *HtmlElement) i32 {
-    const attr = self.asElement().getAttributeSafe(comptime .wrap("tabindex")) orelse return -1;
+    const attr = self.asElement().getAttributeSafe(comptime .wrap("tabindex")) orelse {
+        // Per spec, interactive/focusable elements default to 0 when tabindex is absent
+        return switch (self._type) {
+            .anchor, .area, .button, .input, .select, .textarea, .iframe => 0,
+            else => -1,
+        };
+    };
     return std.fmt.parseInt(i32, attr, 10) catch -1;
 }